Most avrs simply have the internal y-split of the signal, if you get a single sub pre-out a y-splitter will do fine. The avrs with Audyssey (MultEQ) XT32/SubEQ (maybe some older XT on top of the line avrs) features have the ability to set level/delay for two separate sub outputs. I think I've read Pioneer's top of the line avrs with MCACC Pro also claim separate handling of two subs as well, but haven't really delved into those.