You can also run a battery isolator like a Sure Power type. You can get them at most auto parts stores and camping stores for around $40. It keeps one battery from discharging the other while allowing your alternator to charge both batteries at the same time. In one car I ran a red top Optima up front and a yellow top out back near the amp.
I would get something bigger than a 6-800. Go for something with around 75-100 ah. Like a c&d tech you can find them pretty cheap on here if you search the classifieds. I wouldn't run 1 ohm until you get the electrical set up.
your alternator is weak. You must compensate that with more batteries, and bigger wire. You have done the bigger wire, now search for the equation to figure out exactly how much current you will be drawing, and how much current you can make.