I'm not sure about the battery install, but once the car is running, the current is coming from the alt.
I may be wrong, but I think the only reason people run 2 or more batteries is far extended play time while the engine isnt running.
To me, I dont think it's worth it because you wil not get the most from the system because you are not putting out enough voltage for the amps.
As far as alt power, the Impys have 140amp alt. I have 2 Punch 250a2, one puts out 392 watts, the other 404. As long as the engine is idled up, I dont even have a light dimming problem, of course I also run a 1farad cap.