Well I am certainly no expert, but I know a little
Typically a 2 battery setup will add capacity to your system, but not provide "more" power, just power for a longer period.
A two alternator setup does the opposite, adds current but not capacity.
A two alternator system is what you would want if you have constant, engine running, power draw - like in a limo.
A two battery setup is what you would want if you have sudden temporary but high current draws, giving the batteries time to recharge in between.
Bottom line? If you are running on battery power with the engine on, you need a bigger alternator.
Instead of fabbing in a 2nd, I would look at fabbing in a leese neville or other high output alternator, you will not use all the power they can generate, and add a 2nd battery to that (if needed).