as far as vertically biamping your speakers, ie: using one stereo amp per side & having the left channel drive the tweet while the right channel drives the bass/mids, many folks have done this w/good results. the possible cross-talk between channels (bass-treble) in this set-up may be less than the cross-talk between left-right channels in a conventional *horizontal* biamped set-up.
but, there's one thing i'm confused about - ewe say yure using an outboard x-over *before* the amps, then the signal gets routed thru the speakers' x-over? this i definitely wood not do. if ya wanna experiment w/active x-overs, that's fine (tho, again, i'd recommend contacting the speaker mfr as to their recommendations for x-over point/slope/etc). but if ya do this, then ewe should hook up the speaker cable directly to the drivers, & disable the speakers' built-in passive x-over.
good luck, doug