Absolutely not. You should upgrade the battery first, then put in better alternator, then add that second battery. Your alternator is struggling to supply enough current to charge the battery and run the car, so how exactly will the alternator be able to charge TWO batteries and run the car?
so basically everyone is really saying do the big 3 and get a new head unit and upgrade my battery and alt. but i havent really gotten a solid answer as to which i should do first..
maybe do the big 3 first and get a head unit then worry about the alt and bat?