As I discuss in this question, by far the most popular school of Hindu philosophy is the Vedanta school, which bases its tenets on the doctrines laid out in the Brahma Sutras, a work by the sage Vyasa that summarizes and systematizes the philosophical teachings of the Upanishads. (You can read the Brahma Sutras here.) But the Vedanta school didn't always have the dominant position in Hindu philosophy; before the time of Adi Shankaracharya the dominant school of Hindu philosophy was the Purva Mimamsa school, which I discuss here. In contrast to the Vedanta school which is devoted to analyzing the Jnana Kanda of the Vedas, i.e. the Upanishads, Purva Mimamsa focuses on analyzing the Karma Kanda of the Vedas, i.e. the Samhitas and Brahmanas.
One famous Purva Mimamsa Acharya was Kumarila Bhatta, whose shishya Mandana Mishra was defeated in a debate by Adi Shankaracharya. In this excerpt from his Tantra Vartika, Kumarila Bhatta discusses the subject of the same Yagna being discussed in multiple Shakhas of a Veda. For those who don't know, each of the four Vedas comes in multiple Shakhas or recensions as I discuss in this answer. Now the question arises whether someone reading descriptions of the Jyorishtoma Yagna in multiple Shakhas of a Veda will get confused and think there are multiple Jyotishtoma Yagnas. Kumarila Bhatta responds that such confusion will never arise, because a single person is not allowed to read multiple Shakhas of the same Veda:
Objection: "But then one who reads the Texts of two Recensions, could very well know the Jyotishtoma as mentioned in on text to be different from that mentioned in another text[.]" ... [Reply:] This is no argument at all; because the texts of various Recensions are never meant to be read by one man; and the reason for this is that the injunction for Vedic study distinctly points to a single Text being read, specially as it is a general rule that when several things are found to serve the same purpose, they are accepted as optional alternatives; and this would be possible only when the same Action is taken as spoke of in the various texts. That is to say, just as the Class inheres in each Individual, so does the character of "Veda" in each Recensional Text; and then again, in the Injunction "svadhyayo'dyetavyah" (Veda should be read), we find that the Veda is enjoined to be learnt for the purpose of knowing Dharma; and as such we must assign due significance to the singular Number in "svadyayah"; and this distinctly shows that only one Text should be studied.... Then again, if the mere fact of one's being very intelligent could justify his reading of the texts of various Recensions, then the fact of his being exceptionally rich would justify his making an offering of the Yava and Vrihi mixed up.
Kumarila Bhatta's position here is interesting for a couple reasons. First of all, it seems like a position unique to him, since Shabaraswami does not seem to mention this in his commentary on the same part of the Purva Mimamsa Sutras. Second of all, Kumarila Bhatta seems to have violated this rule himself, since he discusses what different Shakhas of the same Veda say!
But my question is, do any scriptures say that you're not allowed to read multiple Shakhas of the same Veda? By the way, I should add that Kumarila Bhatta thinks it's fine to read multiple Vedas (for instance Rig and Yajur), he just has a problem with reading say, two Yajur Veda Shakhas.