The OP mentioned a big stereo , but not much other information .with 2 engines running v belt alternators , that alternator can experience belt slip , and or insufficient output ,trying to recharge 2 batteries and run a truly big stereo.serp belt alt systems typically can offer up to 100 amps output even at medium engine speeds which helps alot. I see alot of failures and voltage drop issues from isolators and prefer to run manual switches to combine left and right battery and left and right combo to house bank and of course never wire main amp.power wires through perk or isolator

I did upgrade my alternators. It's been a while but I think they are rated at 90amps. Bought them from a shop that works only with 12volt stuff rebuilding, selling new, etc. He mentioned that the stock Merc alternators are lucky if they put out 35amps. I do run the separate belts still. I agree that the serpentine system is superior however I am tapped out this season with other purchases so that will have to wait. I ran a 1/0 wire for pos and neg connected directly to the batteries to the location for the amps. Pos goes through a 150 amp breaker (at the batteries) which I have "popped" a couple times. No surprise that I didn't do the math to size it properly. I'm pretty sure that even tho I tried to keep the charging systems separate, I think they are tied together all the time just like if I had the switch set to "combine".

Has anyone tried going to lithium? My only experience with them is in a golf cart but I saved 300lbs and tripled my range. I know bass boat guys are using them a lot for trolling motors and weight savings...

You trying to say an alternator has more turning resistance if it`s pulling a larger load?
Thats not how that works in my experience

Yup, it draws more power (hp) the higher the load.

I have one in my Harley. Spins motor over instant, rarely needs to be on the charger and weighs next to nothing. It feels like an empty battery case.

Where you pick that up, I may need one soon for my street glide.

A lot of the motorcycle wholesalers also have their "brand" as does Harley itself.

I have one from here: https://antigravitybatteries.com/
They also have ones with a restart feature where if dead it has a "reserve".

You've never hooked up a set of jumper cables to a dead car and noticed a decrease in idle speed on the charging car from the load of the dead car?

Energy isn't free, same principles apply whether a car alt or a gen set powering a hospital.
If there was a inverse of power requirements going from mechanical to electrical energy they'd power hospitals with 80 pound Hondas instead of 80,000 pound diesels.

Conservation of energy - Wikipedia