there is no reason to get a seperate alt. You would be running a LOT more power to need that. get yourself a good 250-300 amp alternator and 2 nice batteries and you will be fine.
Start with ONE set of comps. Im prettu sure you will be satisfied. I think you are underestimating the spl of a very good set of properly amped comps. If for some reason you are still not happy, THEN you can add a second set but i highly doubt that will happen.