It is good that you clarify this but I am pretty sure most who responded to your post so far know it already.
I do not own an Onkyo receiver but I know many of the 800 series and up models can do it. My Denon 4308 can do the same but so can the < $1,000 AVR-3310. I suspect it is a common feature nowadays among mid to upper range AVRs, not just Onkyos and Denons.
You are absolutely correct about "Why not?", as long as your goal is to find out if you can get better SQ by separating out the high/low frequency signal paths and not trying to gain more power output.
