There are multiple issues:

(1) Mirrors can briefly have different framebuffers after initialization. This is not really a big deal except that it's subtle and has caused bugs (bug 888553).

(2) Adding new CRTCs to mirrors should likely consider whether there is a format modifier in common, and cause the swap chain to reallocate buffers if necessary, but currently this is not possible. If there is ever a case where modifiers are not consistent things will break.