[Chris Burkard]

As mentioned above, one alternator on one engine charges the house bank and the other engine alternator charges the engine bank. Running one engine only for a long period of time is going to allow one bank to discharge. You should have a 3rd battery switch that says "Emergency Interconnect". If you engage this switch while you are only running one engine, that one alternator will charge both engine and house battery banks.

Keep in mind that if your electrical draw exceeds the alternator output , you are still going to see a discharge. The boat was never designed to operate on one engine for long periods. The alternators output were chosen based on them charging their perspective banks at a moderate load at a moderate rpm. You are asking 1/2 of the system output to charge 100% of the system at the lowest possible rpm no less.

Also keep in mind that by engaging the "Emergency Interconnect", you no longer will have an emergency supply of power should the entire system allowed to be discharged.

You could solve this issue by allowing both engines to run while only one engine in gear, or upgrading to higher output alternators where one can handle the entire vessel load while at idle.

Good Luck!