get a properly rated solenoid and mount it near the aux batt. run a switch to the dash. energize solenoid from te aux batt. when engaged, they run is parallel. switch off, its isolated. use properly rated welding cable and lugs plus leave slack for engine flex.
this sort of shows what i mean and how i set up mine
edit: well not really. got the solenoid from wrangler NW power products. manual switching on/off is a good thing. others use a tap into the oil pressure to energize aug charging circuit, but i like the fully manual option.