The lights are going to dim regardless of how many batterys you have. This is due to the fact that the battery cannot supply the power fast enough when the bass hits hard and fast. If you want to keep your lights from dimming, install a 1 farad capacitor for every 1000 watts in your system. ex. 2000 watts= 2 farad capacitors, 5000 watts= 5 farad capacitors. this is the only way to prevent this. Keep in mind that capacitors have a current draw themselves, adding to the strain on your ALTERNATOR. IMO I would run 1 battery, the best I could find, i prefer optima, and a bigger alternator, and some capacitors. That should yeild optimum results. Usually when people add multiple batterys, they do so because they compete in sound quality competitions, and they compete with the car not running.

peace