Perhaps I'm missing something here, but shouldn't a dual battery setup, equalize it's voltage automatically. With two batteries installed, it's like having one large battery provided they are not isolated. I would think the batteries would simply equalize after time. A normal battery will charge up with anything greater than 12.6 volts right, and the voltage is how you control the rate of charge. If you have a dead battery and jump and start, you will have a high voltage like 15-18 volts charging up the battery, that voltage will slowly creep down back to 13 something over time.
As far as amps, I don't think that has anything to do with it, deep cycle batteries should not be charged at greater than 10amps anyhow, so any alternator will output that and almost all of the time, your alternator isn't putting out hardly anything, until you turn on your headlights which take more than everything else combined. Looking into dual battery setups and deep cycles, I was more concerned with charging the battery too fast, compared to not charging it at all. Perhaps the AGM deep cycles can be charged at a faster rate, but normally deep cycle fast charge is a no no. A normal car battery will charge up in less than an hour usually, they are built to charge up extremely quickly, but should never go below 50-75% charge or damage will start.
