[jclark003]

If one motor starts fine and the other is problematic there is a good chance it is the connection at the starter terminal, engine/starter ground or the battery disconnect switch terminal. If you lean on the starter without using the interconnect you may fine the problem by feel. Usually poor connections get hot quickly when passing a few hundred amps across them. alternatively as Misdirection advised, checking voltage at the starter (while starting) and then working backward across the bank will also help isolate the problem. When the voltage jumps you've passed the point of high resistance. Just make sure charger is off.

I would clean every terminal and ground wire just to be certain. Make sure to turn off charging system and disconnect the batteries before cleaning the starter terminal and exercise caution not to cross the positive and negative terminals on any battery.

Good luck