The easiest and best way is to install the BEP 717-140A-DVSR switch that has been referenced here many times. Then, choose an appropriately sized deep cycle battery to support your house electrical needs. Depending on what batteries you have installed now, you may be able to downsize your cranking batteries to save weight- check your engine manufacturer’s specs as far as required CCA/MCA, etc.
As far as things that will need to be changed out- the switches, obviously, and potentially some of the battery cables depending on how they’re currently configured. You’ll want to check your main circuit protection for your house circuit, which should be a resettable breaker located as close as practicable to the output of the battery switch. If your current setup has inadequate main circuit protection, now is a good time to install a new properly sized breaker. When you remove the old switch, you should have one power cable going to your electrical distribution system and one positive cable going to each engine. You’ll simply install those on the correct locations on the new switch, along with the positive cables from each battery including your new house battery.
I recommend installing a main negative bus bar as well, if you don’t already have one. This cleans up the negative connections at the battery and ensures that all the negatives are properly and effectively tied together.
I don’t have any great pictures at the moment, but here are a couple shots of my battery setup.
