Shoghi Effendi refers to the "twin Covenants" in one of his letters to America:
"Above all, the paramount duty of deepening the spiritual life of these newly fledged, these precious and highly esteemed co-workers, and of enlightening their minds regarding the essential verities enshrined in their Faith, its fundamental institutions, its history and genesis -- the twin Covenants of Bahá'u'lláh and of `Abdu'l-Bahá ... must continue to constitute the most vital aspect of the great spiritual Crusade ..." The Guardian emphasized the importance of strengthening the new Bahá'ís in the Covenant of `Abdu'l-Bahá as well as the Covenant of Bahá'u'lláh:
We do not often speak of the Covenant of `Abdu'l-Bahá, but it is often referred to by the Guardian, and in letters written on his behalf, he emphasizes how important it is to understand and follow its provisions. In each of the following quotes, Shoghi Effendi refers to the Covenant of `Abdu'l-Bahá:
"... the believers need to be deepened in their knowledge and appreciation of the Covenants of both Bahá'u'lláh and 'Abdu'l-Bahá." 
"The Youth must ponder deeply over the significance and implications of the Covenants of Bahá'u'lláh and 'Abdu'l-Bahá, for these form the hub of the Bahá'í wheel, so to speak, the point of unity and strength for all the believers all over the world. Without these Covenants the Divine Protection of God over this new world Faith would not exist. Obedience to these Covenants is the stronghold of all the Bahá'ís, everywhere. The Youth must grasp this fundamental truth, for this will strengthen them in their service to the Cause, as nothing else can or will." 
"Above all, the utmost endeavour should be exerted by your Assembly to familiarize the newly enrolled believers with the fundamental and spiritual verities of the Faith, ... to instil in them a deeper understanding of the Covenants of both Bahá'u'lláh and 'Abdu'l-Bahá ... " Shoghi Effendi stated through his secretary that firmness in the twin Covenants is the very foundation of a Bahá'í's spiritual life:
"The unity and solidarity of its constituent members must be simultaneously reinforced, its roots permanently planted in the soil of the Covenants of both Bahá'u'lláh and `Abdu'l-Bahá ... " 
"The deepening and enrichment of the spiritual life of the individual believer, ... his understanding of the fundamentals of the Covenants established by its Author and the authorised Interpreter of its teachings..." 
"The thing the German believers must do is to deepen themselves in the Covenants of Bahá'u'lláh and `Abdu'l- Bahá. Mere intellectual understanding of the teachings is not enough. Deep spirituality is essential, and the foundation of true spirituality is steadfastness in the Covenant."  The Master promised us "mental tests" and the Guardian promised that these intellectual tests would be safely met by greater knowledge of the Covenant of `Abdu'l-Bahá as well as that of Bahá'u'lláh, and in particular, by studying the Will and Testament of `Abdu'l- Bahá, which is the repository of His Covenant:Since the beginning of the Guardian's ministry, the people who have broken the Covenant have actually broken the Covenant of `Abdu'l-Bahá. Shoghi Effendi referred to "the breakers of Bahá'u'lláh's and `Abdu'l-Bahá's Covenants" on page 58 of "Messages to America," and in a similar vein he wrote of those who have broken the Covenant of `Abdu'l-Bahá:
" ... the believers need to be deepened in their knowledge and appreciation of the Covenants of both Bahá'u'lláh and 'Abdu'l-Bahá. This is the stronghold of the Faith of every Bahá'í, and that which enables him to withstand every test and the attacks of the enemies outside the Faith, and the far more dangerous, insidious, lukewarm people inside the Faith who have no real attachment to the Covenant, and consequently uphold the intellectual aspect of the teachings while at the same time undermining the spiritual foundation upon which the whole Cause of God rests. He feels you and your dear family should do all you can to teach the believers the Will and Testament to strengthen their understanding of its important provisions; for all the authority of the administrative bodies, as well as of the Guardian himself, is mainly derived from this tremendous document."  The Guardian stated through his secretary that what the believers must have "more than anything else in the world" is a deeper understanding of the Master's and Bahá'u'lláh's Covenants, and that without this the teaching work would not "accomplish anything:"
"The Guardian has been considerably disturbed by the inharmony that has arisen in Germany. He feels that what the German Bahá'ís need -- and must have -- more than anything else in the world is a far deeper understanding of the Covenants of both Bahá'u'lláh and the Master. This is the rock foundation without which no sound super-structure can be built. Neither the administration, nor the general teaching work of the Cause in Germany, will progress, or be able to accomplish anything, unless the believers are truly firm, deep, spiritually convinced Bahá'ís. An intellectual grasp of the teachings is purely superficial; with the first real test such believers are shaken from the bough! But once a Bahá'í has the profound conviction of the authority from God, nested in the Prophet, passed on to the Master, and by Him, to the Guardians, and which flows out through the assemblies and creates order based on obedience -- once a Bahá'í has this, nothing can shake him. He, therefore, urges you, and the other members of the N.G.R., to devote as much time as you possibly can, to educating the believers in the Covenant." 
"It is a pity that some of the Western friends, with remarkable naivete, do not grasp the fact that there is absolutely nothing keeping those who have broken the Covenant, whether Bahá'u'lláh's or the Master's out of the Cause of God except their own inner spiritually sick condition.... Unfortunately a man who is ill is not made well just by asserting there is nothing wrong with him!" It lends a whole new dimension to the concept of Covenant- breaking, to realize that in this Day to be declared a Covenant- breaker is to have broken the very Covenant of `Abdu'l-Bahá.
I believe that this is the Covenant of `Abdu'l-Bahá, the appointment of His twin Successors, to whom all must turn:
"The sacred and youthful branch, the guardian of the Cause of God as well as the Universal House of Justice, to be universally elected and established, are both under the care and protection of the Abha Beauty, under the shelter and unerring guidance of His Holiness, the Exalted One (may my life be offered up for them both). Whatsoever they decide is of God. Whoso obeyeth him not, neither obeyeth them, hath not obeyed God; whoso rebelleth against him and against them hath rebelled against God; whoso opposeth him hath opposed God; whoso contendeth with them hath contended with God; whoso disputeth with him hath disputed with God; whoso denieth him hath denied God; whoso disbelieveth in him hath disbelieved in God; whoso deviateth, separateth himself and turneth aside from him hath in truth deviated, separated himself and turned aside from God. May the wrath, the fierce indignation, the vengeance of God rest upon him! And again, the closing words of His Last Will and Testament, and thus His last guidance to the human race:
"O ye the faithful loved ones of 'Abdu'l-Bahá! It is incumbent upon you to take the greatest care of Shoghi Effendi, the twig that hath branched from and the fruit given forth by the two hallowed and Divine Lote-Trees, that no dust of despondency and sorrow may stain his radiant nature, that day by day he may wax greater in happiness, in joy and spirituality, and may grow to become even as a fruitful tree. For he is, after 'Abdu'l-Bahá, the guardian of the Cause of God, the Afnan, the Hands (pillars) of the Cause and the beloved of the Lord must obey him and turn unto him. He that obeyeth him not, hath not obeyed God; he that turneth away from him, hath turned away from God and he that denieth him, hath denied the True One. Beware lest anyone falsely interpret these words, and like unto them that have broken the Covenant after the Day of Ascension (of Bahá'u'lláh) advance a pretext, raise the standard of revolt, wax stubborn and open wide the door of false interpretation. To none is given the right to put forth his own opinion or express his particular conviction. All must seek guidance and turn unto the Center of the Cause and the House of Justice. And he that turneth unto whatsoever else is indeed in grievous error." The Greatest Holy Leaf spoke eloquently of turning to the Master's Covenant:
"Although the towering citadel of God's Cause is upraised on foundations of iron, and His Word is founded on authority and power, and the loyal and firm in His Covenant, through the blessings of the Abha Paradise, stand immovable as the mountains, and are fast- rooted in their love - still, the hurricanes of tests are mighty as well, and from every side comes the thundering roar of violent commotions and bitter trials. From these, at every moment each one of us should beg of God to defend and protect us. "Let us call to mind the clear statements and the warnings revealed by the Blessed Beauty, and the explanations and commentaries of 'Abdu'l-Bahá, particularly as found in His Will and Testament. This Testament was the last song of that Dove of the Rose- garden of Eternity, and He sang it on the branch of the Tree of bestowal and grace. It was His principal gift, indeed the greatest of all splendours that radiated forth from that Day-Star of bounty, out of the firmament of His bestowals. This Testament was the strong barricade built by the blessed hands of that wronged, that peerless One, to protect the garden of God's Faith. It was the mighty stronghold circling the edifice of the Law of God. This was an overflowing treasure which the Beloved freely gave, a goodly and precious legacy, left by Him to the people of Bahá. In all the world, no gift could equal this; no dazzling gem could rival such a precious pearl. "With His own pen, He designated as Guardian of the Cause of God, Shoghi Effendi Rabbani, the Chosen Branch, and made him the 'blest and sacred bough that hath branched out from the Twin Holy Trees,' to be the one to whom all must turn, the centre and focus of all on earth. "In unmistakable terms did He set forth the obligations and elucidated the nature of the institutions of God's Holy Faith. He laid hold of discord's tree and brought it down. He for ever shut the door on conflicting interpretations and views. With every breath ought we to offer praise and thanks to the God of Grace for this bestowal. It is incumbent upon us to read and meditate on the contents of the Will and Testament at all times, and implore God at His Holy Threshold that He will aid us to carry out whatsoever it ordains." 
 Shoghi Effendi, "This Hour, Crowded With Destiny," letter dated August 18, 1949, "Citadel
of Faith," p. 76.
 From a letter of the Guardian's secretary on his behalf to an individual Bahá'í in Germany, The Light of Divine Guidance, Volume 2, page 84.
 From a letter of Shoghi Effendi to the National Youth Committee of the Bahá'ís of Germany and Austria dated 20 October 1949, "The Light of Divine Guidance," Volume 2, page 89.
 Shoghi Effendi, Messages to Canada, pp. 61-62.
 Shoghi Effendi, Letters to Australia and New Zealand 1923- 1957, p. 80.
 Shoghi Effendi, Letters to Australia and New Zealand 1923- 1957, p. 93.
 From a letter of the Guardian's secretary on his behalf to an individual Bahá'í in Germany, "The Light of Divine Guidance," Volume 2, page 83.
 From a letter of the Guardian's secretary on his behalf to Mrs. Anna Grossman, "The Light of Divine Guidance," Volume 2, page 84.
 From a letter of the Guardian's secretary on his behalf to Hermann Grossman, "The Light of Divine Guidance," Volume 2, pp. 83-84.
 From a letter on behalf of the Guardian to the National Spiritual Assembly of the United States dated April 11, 1949, Lights of Guidance, Second Edition, Page 188, #618.
 Will and Testament of `Abdu'l-Bahá, p. 11.
 Will and Testament of `Abdu'l-Bahá, pp. 25-26.
 From a letter of the Greatest Holy Leaf, "Bahiyyih Khanum," pp. 209-211.