On TJ and older Wrangler the tray on the opposite side battery tray is empty. People will run dual battery setups that way. There are even options for all Wrangler models to put dual batteries in the stock location. $100 to $200 on a TJ Wrangler.
Depends on how close the two batteries are to each other.
I mean, I guess you could physically remove the solar battery, and hand-carry it to the ground just in front of the engine compartment, but now the hassle factor is getting rather high...
I mean, I guess it'd be nice to have wired in, but I can't imagine the time-savings would ever catch up to the initial amount of work.