I disagree with Kr4 and pretty much agree with Egoss with the following comments:
1. If the speaker was not designed to be bi-amped (and most are not), but bi-wired (most cases) you do not get one of the primary benefits of biamplification: eliminating the passive crossover network. (Unless, of course, you remove the network.)
2. When the passive crossover network is removed and you setup a "true" biamped system, the differences are like night and day. Try it on one speaker only and compare the two - I think you'll be impressed. (Just beware that it may be difficult or impossible to remove the hi-pass section only of the midrange crossover if the lo-pass is on the same board.)
3. Vertical biamping is an excellent technique, but only applicable if your amps are not "dual mono" designs. The advantage is not correct timing, as was suggested, but increased power, since the higher frequencies will draw less power and each amp can "borrow" power as needed for the lower frequencies. (I'm not an electrical engineer, but I don't buy the timing issue!)
4. I see nothing wrong with using two different amps, but not with the aforementioned vertical configuration.