Well, you are going to want to hook them up in parallel, not in series. Parallel is where the two batteries negatives are grounded, and the positives run to some sort of a selector switch so you can choose one battery or the other, or both at the same time. With this arrangement you only get 12 volts out of it.

Series is where you take the negative of one battery and connect it to the positive of the other battery, then ground only one battery and run the positive lead from the second battery to your load. With this setup you get 24 volts, because the two 12 volts add up to 24 that way.......you do not want the series setup for your car.

Essentially you can do it like we do it on boats, where you either use a manual selector switch, or you can rig it as a "paralleling setup" where you have a toggle switch on the dash and you use Ford starter type relays to electrically link the two batteries together when needed.

What is your reason for wanting to do the two battery routine? Having trouble cranking your engine? The reason I ask is that generally one BIG battery with lots of CCA (cold cranking amps) will start most engines and eliminates a lot of extra weight and hookups that can cause problems.

Just curious.

Don