Best option is to use a charge spliting diode to keep the batteries seperate, otherwise....
Cheap, not very good option - Place a relay (at least 60 amps with 60 amp wiring) between the 2 +ve terminals and set it up to close it's contacts when it sees ignition signal.
Better Option - a voltage sensitive relay (again at lest 60amp relay and wiring) that closes to allow the 2nd battery to charge when the first one is charged.