it will just take longer to drain the battery and your alternator will run all the time and break faster. You need to be able to produce as much power as you will consume on a regular basis. Batteries are basically for storing the power to start the car or to make up for times on higher consumption but if you have lets say a 90 amp alternator and your consistently drawing 120 amps adding a second battery won't over come that - just take longer to drain it. Capacitors are just to supply the amp with a large amount of power for a brief second at a time.
You need to sit and calculate how much power you will be consuming then if its more than your alternator is putting out then you need to upgrade the alternator. A second battery on top of that doesn't hurt but isnt the solution.