Comb filtering is selective canceling. As Alan has pointed out a few times, this happens with any setup with multiple speakers, much less any setup with multiple speakers with multiple drivers.

If you want to avoid comb filtering, use one speaker with a single full range driver in it. See how ya like it...

Mike, since you have two M22s, you're already experiencing the horror of comb filtering.

I am the Doctor, and THIS... is my SPOON!