OK... Here's how I see it:
Vertical bi-amping would involve 2 stereo amps, not necessarily "stacked", but the left channel of amp 1 would drive the tweeters of speaker 1, and the right channel of amp 1 would drive the tweeters of speaker 2. Horizontal bi-amping would have the left channel of amp 1 driving the tweeter of speaker 1, and the right channel driving the woofer of the same.
I've always biamped horizontal, just in case of anomolies or minor differences in the amplifiers or power supplies/ transformers themselves. This would seem to have less impact on the width and focus of the soundstage if one of the amplifiers wasn't quite "on step" with the other. Of course this means that the amplifier driving your woofers is getting a world-class workout, while the high frequency amp is barely warming up.
While we're on the topic...
I'm currently using a pair of Anthem MCA2's to biamplify my NHT 3.3's (horizontal). I'm considering purchasing an Anthem AMP2se to amplify my mid/tweeters, and would appreciate advice and/or suggestions before I embark on this experiment. If things don't balance out well, I'll most likely purchase a SECOND AMP2se to drive the woofers as well.
Monoblocks x 4 would be the way to go, if cash were no object.
My 2 cents.
David.