I have seen a setup like that before, a work around for starting old snow plow trucks.
The whole truck should be stock 6V and the second battery is connected when you push the extra starter button. Like attaching a jumper battery but you don't have to connect the cables.
The problem is that you have to connect a charger to the second battery once and a while because it isn't charged by the truck.
The starting procedure is: key on, stomp starter and then push the button while the starter is down for the boost.
A real asset in the middle of a blizzard.
