The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
7. Or rather there is no (unity of the vidyâs), owing to the difference of subject-matter.
Setting aside the view maintained by the pûrvapakshin, we have rather to say that, owing to the difference "of subject-matter, the two vidyâs are separate.--In the Khândogya the introductory sentence (I, 1, 1), 'Let a man meditate on the syllable Om (as) the udgîtha,' represents as the object of meditation the syllable Om which is a part of the udgîtha; thereupon proceeds to give an account of its qualities such as being the inmost essence of all ('The full account, however, of Om is this,' &c.); and later on tells, with reference to the same syllable Om which is a part of the udgîtha, a story about the Gods and Asuras in which there occurs the statement,' They meditated on the udgîtha
as that breath 1.' If now we should assume 2 that the term 'udgîtha' denotes here the whole act of worship (not only the syllable Om which is a part of the udgîtha), and that (in the passage, 'they meditated on the udgîtha as that breath') the performer of that worship, i.e. the Udgâtri-priest, is said to be meditated upon as breath; our interpretation would be open to two objections: in the first place it would be opposed to the introductory sentence (which directly declares the syllable Om to be the object of devotion); and in the second place it would oblige us to take the word udgîtha (in 'they meditated on the udgîtha'), not in its direct sense, but as denoting by implication the udgâtri. But the rule is that in one and the same connected passage the interpretation of later passages has to adapt itself to the earlier passages. We therefore conclude the passage last quoted to teach that the syllable Om which is a part of the udgîtha is to be meditated upon as prâna.--In the Vâgasaneyaka on the other hand there is no reason for taking the word udgîtha to denote a part of the udgîtha only, and we therefore must interpret it to denote the whole; and in the passage, 'Do thou sing out for us,' the performer of the worship, i.e. the Udgâtri-priest, is described as prâna. In reply to the pûrvapakshin's remark that in the Vâgasaneyaka also the udgîtha and the prâna occur in co-ordination (in the passage, 'He is udgîtha'), we point out that that statement merely aims at showing that the Self of all is that prâna which the text wishes to represent as udgâtri. The statement, therefore, docs not imply the unity of the two vidyâs. Moreover, there also the term udgîtha denotes the whole act of worship (while in the Khândogya it denotes the omkâra only). Nor must it be said that the prâna can
impossibly be an udgâtri, and that on that account our interpretation of the Brihad-âranyaka passage is erroneous; for with a view to pious meditation scripture may represent the prâna as udgâtri as well as udgîtha. And, moreover, the Udgâtri actually performs his work by the strength of his breath; hence the prâna may be called udgâtri. In accordance with this the text says (I, 3, 24). 'He sang it indeed as speech and breath.'--And if we understand that the text clearly intends to convey a difference of matter we have no right to conclude from merely apparent similarities of expression that only one matter is intended to be expressed. To quote an analogous instance from the karma-kânda: In the section relative to the unexpected rising of the moon during the darsa-sacrifice, as well as in the section about the offering to be made by him who is desirous of cattle, we meet with identical injunctions such as the following one, 'He is to divide, the grains into three portions, and to make those of medium size into a cake offered on eight potsherds to Agni the Giver,' &c.; nevertheless it follows from the difference of the introductory passages of the two sections that the offerings to be made on account of the moon's rising are indeed not connected with the divinities of the darsa-sacrifice (but do not constitute a new sacrifice separate from the darsa), while the section about him who is desirous of cattle enjoins a separate sacrificial performance 1.--Analogously a difference in the nature of the introductory clauses effects a difference of the vidyâs, 'As in the case of that which is greater than great.' That means: Just as the meditation on the udgîtha enjoined in the passage, 'Ether is greater than these, ether is their rest; he is indeed the udgîtha, greater than great, he is without end' (Kh. Up. I, 9, 1), and the other meditation on the udgîtha as possessing the qualities of abiding within the eye and the sun, &c. (Kh. Up. I, 6), are separate meditations, although in both the udgîtha is identified with the highest Self; so it is with vidyâs in general. The special features of different vidyâs are not to be combined even when the
vidyâs belong to one and the same Sâkhâ; much less then when they belong to different Sâkhâs.
194:1 From which it appears that the Khândogya enjoins throughout a meditation on the syllable Om which is only a part of the udgîtha; while the object of meditation enjoined in the Brihad-âranyaka is the whole udgîtha.
194:2 Viz. for the purpose of making out that the object of meditation is the same in the Khândogya and the Brihad-âranyaka.
195:1 Cp. Taitt. Samh. II, 5, 5, 2; Pû. Mî. Sû. VI, 5, 1.