This article is from the Car Audio FAQ, by Ian D. Bjorhovde (email@example.com) with numerous contributions by others.
If you only have one line-level set of outputs available, and wish to
power two sets of speakers from a single amplifier, you may be able to
save money by purchasing a two channel amplifier which is stable to two
ohms rather than spending the extra money for a four channel amp. If
you do this, however, you will be unable to fade between the two sets
of speakers (without additional hardware), and the damping factor of the
amplifier will effectively be cut in half. Also, the amp may run hot
and require fans to prevent overheating. If you have the money, a four
channel amp would be a better choice. You would need to add a dual-amp
balancer in order to maintain fader capability, however, but it is more
efficient than building a fader for a two channel amp. If you wish to
power a subwoofer or additional speakers as well, you may want to
purchase a five or six channel amp.