You more than likely won't need a higher output alternator for that setup, but for arguments sake-
adding a seccond alternator is as simple as making a bracket, lining up the belt and running a cable to the battery, that's it.
The main reason to upgrade the alternator on 1000WRMS and smaller systems is to (1) make the alternator lasts longer by going to a larger case with better heat dissapation capabilities, and (2) to get the voltage higher to get better performance out of the amplifiers. If you can maintain 14.5ish volts under a load, the subs are going to be a lot louder than with the same amplifier running at 12.0V.
just my $.02