Not sure where you are seeing that- out-of-box the kit has one battery- on the passenger's side, with the coolant/washer solvent tank on the opposite. If you wanted to model a dual-battery truck you'd need to relocate the tank and put a second battery in its place. That's basically what Ford did on the diesels. Sucky part is, being a Monogram kit, both the battery and tank are molded to the inner fender/core support structure.
I've seen a couple of gas-powered F-series trucks from this body style with dual batteries as well, but I can't tell you for sure if Ford offered dual batteries on gas powered trucks or if those were owner-installed setups.
I heard Ma's Resin was working on a 7.3 IH/Ford diesel, but I don't think it's available for sale yet.