I presume you are not using standard front door speakers, as if they are by putting the on an amp may just blow them up.. However hopefully the info below will help.
you can get single amps that have separate channels.
you headunit should have preamp outputs some time single set (two connector) some time double (4 connectors), you can tell this by looking at the back of your head unit. you will have sets of outputs (1 red and 1 white = 1 channel) . one is left and one is right.
If you on;y have one set you can buy adaptors that plug onto the end of the phono cable to convert to run extra channels.
(apologies if it all seem unclear, will make more sense if you look at the back of the Headunit)