Battery Minder has this
device that allows two 12 volt batteries to be charged by one Battery Minder. I have one on order and with this I will be able to charge my two 6 volt house batteries and 12 volt engine battery with one Battery Minder. My Battery Minder is rated to charge three batteries at one time.
Currently I use two Battery Minders, one for house batteries, and one for my engine battery.
Battery Minders have a mode to remove battery
sulfation. Sulfation, a build-up of lead sulfate crystals, is the number one cause of early failures of lead-acid, sealed AGM or flooded (wet cell-filler caps) batteries. Before I started using Battery Minders I had to change my boat and RV batteries very often.
Or you can follow
these instructions on connecting to more than one battery.
Battery Minders cautions to use a charger that is designed to charge more than one battery at a time.
Safe travels.
JD