I have a 2000 382 with 500efi motors and 2 batteries. For main battery switches, I have an on/off dial for each battery, and a key switch for running the 2 batteries in tandem if one is dead. If I see it correctly, 1 battery is strictly for starting 1 motor, and the other is for starting the other motor and all aux power. If I drain the battery that supports the aux power, my understanding is that I can use the key switch to start that motor off both batteries, but that does not seem to work. My one battery was dead in the driveway last weekend, but when I turned the key to run with both batteries, it still wouldn't turn over even though the other motor turned over and started fine. Is there a trick like the other motor needs to be running, dead battery switched to off, charger circuit turned off, etc? Maybe my batteries are wired incorrectly?

Did you have all three switches on? Turning on the good battery and the combiner switch might not work if you don't have the dead switch on as well.

Yes, with all 3 switches turned on, 1 motor turns over fine and the other on the dead battery does not. I would assume that in this case each motor would turn over the same if operating properly. When charged, everything works normally.

Maybe the battery switch itself is dead. Have you tried checking the battery with a voltmeter? Maybe it's good...

Could be a cable (power or ground) as well.

I think I remember seeing an integrated breaker or something next to the center switch.....like a pop out button or something?

If not.....ignore my post.

I run with these 2 batteries all the time, it just came up the other day when I accidentally left the stereo in standby and the amps stayed on and drained the one. I'll check for a breaker and maybe pull off that switch plate to see what's going on. I'm assuming there isn't a special way to wire the batteries?

Perhaps the switches were rewired at one time? Follow the power wires from the starters; do they both go to the same switch as they should? If so, it could be your starting relay's (usually built into the starters) could be wired wrong up front.