A 2-channel amp has outputs for 2 speakers (or subs), a 4-channel amp has outputs for 4, and a mono has one ouput. A mono amp would be better for 2 subs.
you can bridge a 2ch amp for 1 sub...or bridge 2 subs into a mono amp...a 4ch amp can also be bridged but i prefer them for components, etc....but the amp to use is dependant on the sub