The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
59. There is (restriction to) option (between the vidyâs), on account of their having non-differing results.
The difference of the vidyâs having been determined, we now enter on an inquiry whether, according to one's liking, there should be cumulation of the different vidyâs or option between them; or else restriction to an optional proceeding (to the exclusion of cumulation). For restriction to cumulation (which might be mentioned as a third alternative) there is no reason, because the separation of the vidyâs has been established.--But we observe that in the case of the sacrifices, agnihotra, darsapûrnamâsa and so on, there is restriction to cumulation (i.e. that those sacrifices have all of them to be performed, not optionally one or the Other) although they are different from each other.--True; but the reason for the obligatory cumulation of those sacrifices lies therein that scripture teaches them to be of absolute obligation. No scriptural passage, on the other hand, teaches the absolute obligatoriness of the vidyâs, and it cannot therefore be a rule that they must be cumulated.--Nor can it be a rule that there must be option between them, because a person entitled to one vidyâ cannot be excluded from another vidyâ. It therefore only remains to conclude that one may proceed as one likes.--But--an objection is raised--we must rather conclude that option between them is the rule, because their fruits are non-different. For vidyâs such as 'He who consists of mind, whose body is prâna;' 'Brahman is Ka, Brahman is Kha;' 'He whose wishes are true, whose purposes are true,' have all of them equally the obtaining of the Lord for their fruit.--This does not affect our conclusion; for we see that it is allowed to proceed as one likes also with regard to certain sacrificial acts which are the means of obtaining the heavenly world, and thus have all of them the same result. It therefore remains a settled conclusion that in the case of vidyâs one may proceed as one likes.
To this we reply as follows. There must be option between the vidyâs, not cumulation, because they have the
same fruit. For the fruit of all of them is the intuition of the object meditated upon, and when this object, e.g. the Lord, has once been intuited through one meditation a second meditation would be purposeless. It would, moreover, be impossible even to effect an intuition through the cumulation of several meditations, since that would cause distraction of attention. And that the fruit of a vidyâ is to be effected through intuition various scriptural passages declare; cp. Kh. Up. III, 14, 4, 'He who has this faith and no doubt;' Bri. Up. IV, i, 3, 'Having become a god he goes to the gods,' and others. Also Smriti-passages such as Bha. Gîtâ VIII, 6, and others.--One therefore has to select one of those vidyâs the fruit of which is the same, and to remain intent on it until, through the intuition of the object to be meditated upon, the fruit of the vidyâ is obtained.