I guess you are going to use them in your car?
To avoild another post, I assume that Yes. In that case, the car's alternator will be taking twice load.
Installing 2 or more batteries are very common in the car audio world, but except if you are going to use the audio system with the engine stopped for long time, it's more a problem than a solution, again, due to alternator overloading.
If the headlights go dim with strong bass, get a capacitor or upgrade the alternator.