Re: 12 volt batteries
It looks like this and is called two batteries wired in "parallel". When you do this, you need to understand that if one of the two batteries is significantly weaker than the other, it will suck the life out of the other battery until the two are equal. When I say "weaker" I mean older, not able to maintain its original capacity, etc. Both batteries should ideally be the same style and capacity (amp/hr rating). When charging those batteries, the time it takes will be roughly double what it takes to charge one of them.
Here is another approach using an inexpensive dual battery switch which allows selection of BAT 1, BAT 2, BOTH, or OFF.
