Sorry if this is not on topic, but I do own the Marantz av10 and amp10. I'm told I can only biamp my two front floor speakers.
As I responded in post#32, and HD's post#35, you can bi-amp any channel you want as long as you have the spare power amp channels. In re-reading your post, I realized you may be referring to the AV10, not the AMP10. If that is the case then I think you may be talking about the AV10's bi-amp assignment feature, that allows you to b-amp the front left and front right channels without using a splitter.
It is just a convenient feature, it is nice that they do the split internally via relays, but electrically speaking it is the same as using an external splitter.
As HD said:
"you could do it to any pair of channels (or just the center channel) by splitting the signal from the appropriate pre-outs before sending it to your amp."
Electrically, therefore sonically, the results will be the same as the AV10's front L/R channel bi-amp assignment feature. Marantz did not offer the same feature for the other channels most likely because of the associated cost and complexity of just a convenient feature that very few people would use anyway, so it makes sense to not force every user to pay so much more for things they don't need, or won't ever use.