The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
63. On account of the rectification.
The passage, 'From the seat of the Hotri he sets right any mistake committed in the udgîtha' (Kh. Up. I, 5, 5), declares that, owing to the might of the meditation on the unity of pranava and udgîtha, the Hotri sets right any mistake he may commit in his work, by means of the work of the Hotri.
Now, as a meditation mentioned in one Veda is connected (with what is mentioned in another Veda) in the same way as a thing mentioned in another Veda, the above passage suggests the conclusion that all meditations on members of sacrificial acts--in whatever Veda they may be mentioned--have to be combined 1.
282:1 A 'thing' belonging to the Rig-veda, viz. the pranava, is, according to the Khândogya-passage, connected with the Sâma-veda meditation on the udgîtha. Hence meditations also which belong to different Vedas may be combined; for there is no difference between them and things as far as connexion is concerned.