Ok i have a few statements and questions.

As Fusions site no longer exists and i can't see the config, are there one set of binding posts per module or two? Crossovers?

Ignore the bi-amping reference

Install an isolator or a perko at the minimum and run both batts. You need to get the stereo gear off the starting circuit. Two amps draw a lot of juice and this will insure you have juice for the stereo and juice to start as well as protect circuits. You can do all of this on one batt but it's not the best way to do it.