Psyprof1 has some good info here. I'll second his suggestion that we first look at the application - depending on the nature of the second zone, you might want to go anywhere from a little Sonic Impact T-Amp or two ($35 each plus AC adapters) to a used stereo amp ($150+) capable of driving both pairs of speakers to a used receiver or five-channel amp ($200 to $500+) driving both pairs of speakers independently to a larger, new amp of some sort. If it's a typical second zone application, I suspect that a used stereo amp might be a good fit as long as the speakers in question are rated for 8 ohms so the combined load doesn't get much below 4 ohms.