it's more like you
might get phase issues, and more with some cab combos than others, and it might be enough to bother you or it might not.
the mixed stack is usually "close enough for rock and roll" and will sound bigger and louder than either individual cab by itself. that alone makes it good enough for lots of people.
the
only point here is that a matched pair of good cabs will definitely
not have any problems like that, and given equal quality/size/power cabs to the mixed stack will
more likely sound better.
(and did i call it on the first page or what? over one weekend and almost 200 posts later we've got stuff about hitler and other stuff about information theory and the psychological effect of sine waves, this thread has behaved perfectly true to expectations

)