I was reminded by Krazeehorse that I posted something along this line a while back.
You can hook batteries up in series all day long, but when you start hooking batteries up in parralell you start asking for problems.
This is due to the difference in something called potential which simply spoken is one battery will have more charge than the other.
If the batteries are left connected and not used one will try and charge the weaker. It would seem they would reach equilibrium at some point but they don't. The weaker all of a sudden becomes the stronger and the charging cycle starts over. After a period of time both are dead as a hammer.
There are some isolators out there if you want to spend the money cause they can get expensive.