Should I always address both PhD supervisors in emails?
I think that asking won’t hurt. But that said, just because two PIs are jointly advising a student on a degree does not entitle one to be privy to all exchanges with the other. Or put another way: just because you have two advisors doesn’t mean you lose all rights of privacy when communicating with either one of them. Of course sometimes information concerns both, so an email addressed to both is appropriate.