>work in harmony
Both (btw. which of those Garamonds?) differ in x-height, construction principles of the glyphs, dynamic vs. static appearance, etc. To prove that try to put them together in one line of text and look at the result – it doesn't feel right in my opinion. But if there is enough space between them, difference in size and weight, etc. then the combination could work.

I agree with poms - I personally think it looks weird, but these things are a matter of taste. If you like it, then use it :-)

Why not?

It's just that every combination of typefaces has its own aesthetics and to get the intended feel you need to mix a proper pair. And, of course, some combinations do not work, and some work only in certain contexts.