Nuforce STA200

Maybe I’m overthinking this-  if one amp’s R and L channel play thru one speaker, and the other amp does the same thing thru the other speaker, do you get the same stereo separation as if you were using one amp?

It doesn't make any difference as your perception of "imaging" is coming from the speakers.  If it makes you feel better then try it both vertical and horizontal and see if you hear any difference.  If you speakers don't need the extra muscle then don't bother.

+1 about imaging from the speakers (including the Xovers). If you might be looking to try to go as "dual mono" as possible for the best theoretical stereo separation with a pair of stereo amps then biamping vertically would be the way to go.. do that first but really best if you try it both ways as recommended by jackd.

The only possible disadvantage is that any variations in gain (unlikely) between the 2 amps will make one side slightly louder and may skew the soundstage when you vertically biamp.

kbuzz, this may be too late, but I saw your post about using a tube preamp. I bought the STA200 and had an old Audible Illusions tube preamp and there was a lot of noise. I bought a Rogue RP1 preamp which I picked partly because it has a low gain of 7 db, and that combination worked well.