I finally finished this project and thought I'd post some details in case anyone else is interested in fitting a similar system.
As I stated earlier, my primary desire was to have a backup battery available to start and operate the vehicle if the main battery became discharged. My secondary desire was to have the ability to combine the two batteries for winching.
The Hellroaring Battery Isolator/Combiner looks to be high quality; each component appears well made and is nicely finished. (You can see the isolator on the driver's firewall side of the battery.)
Inside the cab there is a small remote switch which is used to change between the 3 available modes: "auto" the charging/isolation mode, "on" the combined mode, and "off" the disabled mode.
I originally wanted to use two Optima D31M Blue Top batteries, but there was not enough room. So I decided on two Odyssey PC 1500's instead. These fit (just) within the stock battery location, although it was necessary to fabricate a new battery tray and mounting system (using only existing, stock holes of course).
Parts and materials ran nearly $1,000 for everything. I worked with Flying Circus English Cars, in Durham, NC, to perform the fabrication and installation. They really did a great, quality job - I'd recommend their services to anyone in the Raleigh/Durham area.