You asked what you need.
You need a short, heavy-duty cable to connect the two GC-2 batteries the way others have described. I found a 15-inch (or so) 000 cable with the appropriate battery-connection tabs already made. I think I found it at Tractor Supply. It was much less expensive than having my local RV shop make one, which they would have been willing to do.
You need a way to encase the 2 batteries. I opted for two separate battery cases, each designed to hold one GC-2 battery. I was able to mount them on the space where I had previously had one 12V case.
I also wanted a way to lock them down to prevent theft. I made an aluminum locking strap that surrounds the entire 2-battery-box setup.
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