Which battery to connect winch too?

John,
A winch should be connected to the main starting battery. The reason for this is that the main starting battery is charged from the alternator first. The battery and wiring are designed to take the high amps that are required to run a winch.
I would keep the winch connected to the same battery.
There are a few tips on winching,
Run the motor at about 1200-1500rpm to keep the charge up to the battery
Only use the winch in short runs, 10-15 seconds winching 15-20 off so that the battery can recover
Always run the winch cable out off the drum as much as possible and run it through a snatch block. This will reduce the amount of effort required by the winch and therefore less draw on the battery.

Wayne

Hi John
Just make sure that the leads connecting the two batteries are as big or bigger than the winch cables
Ray

If its a factory setup then the cables connecting the two batteries will be the corrrect rating so it's not going to matter what battery you connect the winch to as they are in parallel.

Leroy

Place the positive on positive on one battery and the earth to earth on the other!because your batteries are wired in parallel, I hope the connecting wire is rated as suitable!