What he said
Actually, if you leave them connected in series, you can put a float charger on them and they will be fine all winter. If you don't have a float charger, just trickle-charge them for a few hours once a month and they will be fine....
The BIGGIE is to have them connected in series (+ to - between the batteries and put the charger on the + of one battery and the - of the other)
It's easy if you aren't paying attention to connect them in parallel and if you apply 12 VDC to them, it'll overcharge them and possibly cause them to not be able to recharge, so be careful how you connect them !!!!!