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.