I have a dbx BX-3 power amp (early 80's) that has a hook-up diagram for a "dual 2 channel system" Two pre-amps, up to eight speakers. I've never done it, but a friend who did, used it basically as you would a modern day second zone. One pre-amp controlled the stuff in the family room (four speakers), and the other controlled a dedicated listening room (two speakers). Didn't lose anything on either system, of course both pre-amps were the same, dbx CX-3's.