The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
2. (If it be said that the vidyâs are separate) on account of the difference (of secondary matters), we deny that, since even in one and the same vidyâ (different secondary matters may find place).
In spite of the preceding argumentation we cannot admit that the different cognitions of Brahman are equally intimated by all Vedânta-texts, because we meet with differences in secondary matters (guna). Thus the Vâgasaneyins mention in their text of the knowledge of the five fires a sixth fire ('And then the fire is indeed fire,' Bri. Up. VI, 2, 14), while the Khandogas mention no sixth fire but conclude their text of the pañkâgnividyâ with the express mention of five fires ('But he who thus knows the five fires,' Kh. Up. V, 10, 10).
Now it is impossible to admit that the cognition of those who admit that particular qualification (i.e. the sixth fire) and of those who do not should be one and the same. Nor may we attempt to evade the difficulty by saying that the sixth fire may be tacitly included in the vidyâ of the Khandogas; for that would contradict the number 'five' expressly stated by them.--In the colloquy of the prânas
again the Khandogas mention, in addition to the most important prâna, four other prânas, viz. speech, the eye, the ear, and the mind; while the Vâgasaneyins mention a fifth one also, 'Seed indeed is generation. He who knows that becomes rich in offspring and cattle' (Bri. Up. VI, 1, 6).--Now a difference of procedure in the point of addition and omission effects a difference in the object known, and the latter again effects a difference in the vidyâ, just as a difference in the point of material and divinity distinguishes one sacrifice from another.
To this we make the following reply.--Your objection is without force, since such differences of qualification as are met with in the above instances are possible even in one and the same vidyâ. In the Khândogya-text a sixth fire is indeed not included; yet, as five fires, beginning with the heavenly world, are recognised as the same in both texts the mentioned difference cannot effect a split of the vidyâ; not any more than the âtirâtra-sacrifice is differentiated by the shodasin-rite being either used or not-used. Moreover, the Khândogya-text also actually mentions a sixth fire, viz. in the passage, V, 9, 2, 'When he has departed, his friends carry him, as appointed, to the fire'. The Vâgasaneyins, on the other hand, mention their sixth fire ('and then the fire is indeed fire, the fuel fuel,' &c.) for the purpose of cutting short the fanciful assumption regarding fuel, smoke, and so on, which runs through the description of the five fires with which the heavenly world and so on are imaginatively identified. Their statement regarding the sixth fire (has therefore not the purpose of enjoining it as an object of meditation but) is merely a remark about something already established (known) 1. And even if we assume that the statement about the sixth fire has the purpose of representing that fire as an object of devout meditation, yet the fire may be inserted in the vidyâ of the Khandogas without any fear of its being in conflict with the number five mentioned there;
for that number is not an essential part of the injunction 1, but merely makes an additional statement regarding something known already from the text, viz. the five fires with which the heavenly world and so on are identified 2. Similarly nothing stands in the way of some additional qualification being included in the vidyâ concerning the colloquy of the prânas and so on. The addition or omission of some particular qualification is unable to introduce difference into the object of knowledge and thereby into the knowledge itself; for although the objects of knowledge may differ partly, yet their greater part and at the same time the knowing person are understood to be the same. Hence the vidyâ also remains the same.
188:1 Viz. the real fire in which the dead body is burned and which is known from perception.
189:1 I.e. the Khândogya-text contains no injunction that five fires only are to be meditated upon.
189:2 So that there stands nothing in the way of our amplifying our meditation by the addition of a sixth fire.