The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
66. And because (scripture) shows it.
Scripture moreover shows that the meditations do not go together, viz. in the following passage, 'A Brahman priest who knows this saves the sacrifice, the sacrificer, and all the priests' (Kh. Up. IV, 17, 10). For if all meditations were to be combined, all priests would know them all, and the text could not specially announce that the Brahman priest possessing a certain knowledge thereby saves the others.--The meditations may therefore, according to one's liking, be either combined or optionally employed.