Simple 2 engine/2 battery/1 switch wiring
 

I need some help with a simple and practical way to wire my twin inboard setup. I currently have two batteries and one switch. The grounds are connected together and wired to a single power post that connects to each block. The positive sides are on a switch 1,2,Both then wired to a positive power post that connects to the both the starters.

How should I wire the alternators to charge these batteries without having to redo this whole setup? I'd prefer not to have to go buy more switches/combiners, however I do want the boat to be safe and reliable.

That seems a strange setup. If both motors are tied to the one swich, and the switch merely selects batteries, then there isn't any way to isolate one side from the other. I don't think having both alternators working the same circuit is advisable, as which ever regulator is set higher is going to do all the work.

I would just go ahead and buy an additional switch. For $50 or whatever it is, you should be able to take the lead from one battery and one engine, and divorce it completely from the other side.

You could just get a on/off switch for the new one, and with one additional heavy cable, wire it so that the "Both" position on the old switch cross-connects the two sides for extra boost/one side dead starting capability. This would be minimal investment for a reliable setup.