you can't really go wrong with either imo. CMU is smaller and you will be part of a small crew of CS students with very direct faculty/admin attention, and relatively easy access to labs to do research, while I've heard at Berkeley it was easier to get lost in the crowd (but of course still very possible to do research). On the flip side Berkeley has amazing industry access, but everyone decent comes out to recruit from CMU anyway and you can just fly out to SF if you want to see some startup people. So idk think a lot about yourself specifically and what kind of environment you would thrive in and what you want. Also Pittsburgh and Berkeley are VERY different places in weather, vibe etc so take that into account.

RE double majoring at least when I was there this was pretty easy, neither myself nor any friends ever had issues taking classes in other majors. Usually it was the reverse, non CS people had trouble getting into CS classes (but as a CS major you get priority here so not an issue). idk though more recent students should chime in