To keep your volts a 12v you have to go positive to positive. Negative to negative.

If you go neg to pos , neg to pos, you will make it 24 volts.

Then draw your power from positive the first battery and negative the second battery. This will draw power through both batteries evenly.

Same as to charge the system. Place the leads positive on one battery and nagative on the other battery.

If unsure Battery World will give you a free chart on the basics of joining and charging batteries.

Cheers Grant

Might pay to look at something like the bep vsr circuits before rushing into battery switching too ;)

http://www.bepmarine.com

The first from the left is the most basic and regularly utilised on a boat. The third from the left is essentially the same thing, but utilises a Voltage sensitive relay instead of the Cole-Hersey. This is a 'better' way to do it as it disconnects the starting battery when the system voltage reduces to 12.7V. There are heaps of different diagrams on that site that are basically the same thing.

Look up some of the site sponsors, I know that there are couple of really good Auto Sparks here.

Good luck

http://www.google.com.au/search?q=dual+battery&hl=en&prmd=ivns&tbm=isch&tbo=u&source=univ&sa=X&ei=7iXmTejBJIm-uwPz6YT4Cg&ved=0CFcQsAQ&biw=1259&bih=771

the way till said is the most common and simple way to do