I don't think there is a good way for you to do this unless you have balanced outputs for all five channels. For balanced outputs, there is a unique way to use two amps. The idea was unique enough to be granted a patent on June 22, 1999. (Patent# 5,914,636)
http://patft.uspto.gov/netacgi/nph-Parser?patentnumber=5914636
Go here and read the short abstract and you will understand this method.
http://patft.uspto.gov/netacgi/nph-Parser?patentnumber=5914636
Go here and read the short abstract and you will understand this method.