The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut [1896] at sacred-texts.com
6. If it be said that (the udgîtha vidyâ of the Bri. Up. and that of the Khând. Up.) are separate on account (of the difference) of the texts; we deny this on the ground of their (essential) non-difference.
We read in the Vâgasaneyaka I, 3, 1, 'The Devas said, well, let us overcome the Asuras at the sacrifices by means of the Udgîtha. They said to speech: Do thou sing out for us.--Yes, said speech,' &c. The text thereupon relates how speech and the other prânas were pierced by the Asuras with evil, and therefore unable to effect what was expected from them, and how in the end recourse was had to the chief vital air, 'Then they said to the breath in the mouth: Do thou sing for us.--Yes, said the breath, and sang.'--A similar story is met with in the Khândogya I, 2. There we read at first that 'the devas took the udgîtha, thinking they would vanquish the Asuras with it;' the text then relates how the other prânas were pierced with evil and thus foiled by the Asuras, and how the Devas in the end had recourse to the chief vital air, 'Then comes this chief vital air; on that they meditated as udgîtha.'--As both these passages glorify the chief vital air, it follows that they both are injunctions of a meditation on the vital air. A doubt, however, arises whether the two vidyâs are separate vidyâs or one vidyâ only.
Here the pûrvapakshin maintains that for the reasons specified in the first adhikarana of the present pâda the two
vidyâs have to be considered as one.--But, an objection is raised, there is a difference of procedure which contradicts the assumption of unity. The Vâgasaneyins represent the chief vital air as the producer of the udgîtha ('Do thou sing out for us'), while the Khandogas speak of it as itself being the udgîtha ('on that they meditated as udgîtha'). How can this divergence be reconciled with the assumption of the unity of the vidyâs?--The difference pointed out, the pûrvapakshin replies, is not important enough to bring about a separation of the two vidyâs, since we observe that the two both agree in a plurality of points. Both texts relate that the Devas and the Asuras were fighting; both at first glorify speech and the other prânas in their relation to the udgîtha, and thereupon, finding fault with them, pass on to the chief vital air; both tell how through the strength of the latter the Asuras were scattered as a ball of earth is scattered when hitting a solid stone. And, moreover, the text of the Vâgasaneyaka also coordinates the chief vital air and the udgîtha in the clause, 'He is udgîtha' (Bri. Up. I, 3, 23). We therefore have to assume that in the Khândogya also the chief prâna has secondarily to be looked upon as the producer of the udgîtha.--The two texts thus constitute one vidyâ only.