capacitors will help with your dimming, but a second battery isn't going to help you there.... Adding another battery is only going to put more of a load on your alternator, because now you have to charge two batteries.... extra batteries will help you run your system for longer when the car is off, but when the car is running, your alternator is the main source of power... A capacitor will help smooth out the "peaks and valleys" of your current draw, and that will usually stop your headlights from dimming....
Just like a battery, however, a capacitor is going to add another load to your system as well, because a capacitor is always going to add resistance to the circuit, and is never going to be 100% efficient, so overall, there is no substitute for a high output alternator. If you put in a cap and you are still noticing your lights dimming, or your voltage dropping, then an alternator is your only solution.