I take that back. You can run two speakers off the same amplifier channel, provided that you do the correct impedance matching. There's a whole system for doing this sort of thing. Basically, you end up needing twice the amplifier power than what's shown on the box (since each channel is now driving two speakers instead of one), but it works. It's a common technique. Most likely covered in detail at one of these places (if any of those old links still work).