Anyone prefer horizontal bi-amping their monos?

What do you mean?
Two monos per side into biwired speakers?

or is a mono a speaker?

2 amps total

I think many people run a mono for each speaker.
If nothing else you end up getting a short speaker cable.

And I suppose save some rack space if one can hid them behind the speaker.
And also those cable lifters go out the door when the speaker cable is going vertical.

You sure got me a scratchin’ my head with your question. To bi-amp  a pair of speakers you need 4 mono amps. Horizontal and vertical bi-amping is used when one uses two stereo amps when bi-amping speakers. Kapish?