Its harder to have load options for door speakers seeing as they are all single voicecoil. If it was me I'd just buy a 4ch amp and be done with it. Unless you are trying to run 2 sets of components, and are trying to power 2 tweeters and midbass. In that case I'd buy a 2ch amp, run the midbass on the 2ch amp, then run the tweeters off the headunit.
do not run the tweeter off the head unit, run the crossover off of the amp u choose becasue it will do its job and crossover the freq for the comp system. dont seperate a system thats ment to be run together.