The most important factor here may not be whether or not the two majors of interest – in this case, theater and English – are in the same division of the university.
At most UCs, there is something like a “College of Letters & Sciences” or a “School of Humanities”, which includes both English and Theater. In this situation, an English/theater double-major should be relatively straightforward, because both majors are in the same branch of the university, which means that they should share the same general degree requirements.
At UC Irvine and UCLA, on the other hand, theater is in a separate school from English. UCI has theater in the School of Arts, and UCLA has a School of Theater, Film, and Television. In this case, it might be harder to earn a double major in English/theater, because you would have to meet the general degree requirements for two different divisions of the university.
It looks like the UCLA School of Theater, Film, and Television doesn’t allow double majors. Not sure about UCI.
http://catalog.registrar.ucla.edu/ucla-catalog2017-154.html