I was going to go with the side by side arangement like that. But the second one ends up right above the exhaust manifold, I was afraid the heat may kill it.
On the splash panel where I ended up putting it I cut out a section of the panel, so the battery sticks into the wheel well about 1/3 (the wheel will never come up that high,) then built the tray - solid on the bottom to keep mud and dirt from coming up, then braced the #### out of it.
It's all welded in, so far it hasn't shown any signs of weakness - no cracks or anything. But Paul you may drive yours harder.
The other thing is charging it - I didn't like any of the splitters on the market, most don't charge the battery(s) fully.
I simply tied BOTH (+) posts together using the side terminals. Then connected the regular battery cable to one of the top (+) terminals.
Then I used quick disconnect cable ends on the ground sides
Normally I leave only one ground cable connected at a time - that lets it run 100% on that battery. Every time I check the oil, I disconnect the ground being used, and connect the other one to the other battery. Then the other battery gets the excercise.
In case I ever need both for double the cranking power, I'd simply connect both grounds, then they are in parallel.
The benefit of disconnecting the ground side rather than the (+) side is the hanging cable is just ground, safe - no fires!
I don't like the idea of mounting it inside the vehicle - remember as a battery charges and discharges, Hydrogen gas is released - explosive. (Ever see a battery blow up?) And as it charges and discharges, Sulpheric acid fumes come out. You've seen how the battery tray gets corroded, not a good idea to have those fumes inside - or breathe them.
And - in a rollover a regular battery is going to spill - not fun on the back of your neck -- I speak from experience on that with a dune buggy.
I've heard quite a few negative things on the Optima - are they just rumors of facts?