Sopper -
Assuming the two batteries are the same make/model/size/age, then yes, two in series will be just fine. If the two batteries were different, then they would charge at different rates, meaning that one would undercharge and the other would overcharge.
As for storing the batteries through the winter, there are at least a couple different answers. Mine is this.
A discharged battery will freeze and break at cold temps. But a low state of charge will hurt the battery regardless of anything else. So never store a discharged battery, regardless of temperature, and never let it become discharged during storage.
Is it that simple? Well, it is important to remember that if a battery is fully charged, and then disconnected from everything, it will still discharge via
self discharge. It is self-discharge that is your enemy.
A good source of information is
www.batteryfaq.org (click on
Car and Deep Cycle Battery FAQ, and scroll down to section 13.2). There you will find
"Batteries naturally self-discharge 1% to 60% per month (depending on the battery type and temperature) while not in use.... Cold will slow the process down and heat will increase it up....Store [the battery] in a cold dry place, but not so that it will freeze"
This site has a lot of other good information (such as the concrete floor myth), and is worth checking out. But at the bottom line, assuming that you start with a fully charged battery, then cold is your friend, not your enemy.
Incidentally, I can attest that this cold-storage approach works. When I leave Maine for the winter every year, I shut down all heat in the house, and leave my mini-pickup truck in the unheated garage. In this part of Maine, temperatures often dip to zero during the winter, though seldom lower. I don't have a charger because the power to the house is off. All I do in the fall is wash the top of the battery, charge it, then disconnect it and leave it in place. When I get back in the spring, I reconnect the battery, and it has never failed to start the truck's engine immediately. I've done this for 12 years with never a problem.
Bill