Re: Replacing 12 volt batteries with 6 volt
"can 2 batteries be connected in series and parallel at the same time and is that a good idea."
No, that would be called a mess or a short circuit and start a fire or battery explosion. You can connect two batteries in series OR in parallel but not both at the same time.
One way doubles (combines both) the output voltage, the other doubles (combines both) the output amperage.
Be careful with the batteries, Larry. Most folks don't realize the a single 12-volt car battery can put out over 3000 amps into a short circuit, and if your wedding ring is part of that short, it will amputate your finger. A significant number of folks are blinded in the US every year when they literally blow up their car batteries while trying to jump start cars, too.
A 1/5A charger will eventually charge them, and is a good way to keep them in shape whilenot in use, but once you read the book I suspect you'll want something better.