Use my Denon 4200w to power my highs on a speaker and an separate amp to power my lows vice versa?
See the section in your manual for
bi-amp connection of front speakers. You'd basically configure the "amp assign" setting to bi-amp, then connect the mids of your mains to the Front L/R terminals. Connect the Surround Back preamp outs to an amp, then connect your tweets to the amp. I think. If that doesn't work, then try it the other way around -- connecting the mids to the "Assignable" speaker terminals and use the Front L/R pre-amp outs to the amp for the tweets.
If you're doing this because you like hacking things, because you already have the amp, and just because you feel like it, then I do not object. Experimentation is part of the adventure. Go for it!
On the other hand, if you're doing this because your tweets are too hot or are not hot enough, then you need better speakers. Your intended kludge will potentially allow you to squash a peak in the response, but you'll introduce valleys elsewhere. And for a speaker that's so poorly designed that you would resort to bi-amping as a means of fixing its response, you'll probably have other problems with edge diffraction, lobing, poor off-axis response resulting in unpleasantly colored first reflections, exposing cone break-up, or some other factor resulting in an otherwise generally mediocre sound.
