I am kinda in the same pickle, i have a 4ch and a monoblock

My headunit only has two outputs as well =[.

What i ended up doing (not because i wanted to) was connecting one set to my 4ch and the other set to my monoblock. Why? because for some crappy reason the inputs on my 4ch (for the rear) introduce awful alternator whine when they're connected.

Good news is it sounds just as good with only one input.

Why?

Most music you listen to is only in stereo, i dont think you have any 4.1 surround sound music cds... i know i dont. =[