FWIW, you could by them that way back in the square body days. Unfortunately replacement parts for the stock setup are non existant, and it doesn't seem like a large percentage of sales had the option.
I saw this guy from Australia add an aftermarket tank, just a generic auxiliary tank and installed a 3 way solenoid valve so he could flip a switch from inside his cab, he said that he had to get a slightly smaller tank than his stock tank and he said something about letting your first tank almost completely drain and then swap over to the aux tank, and then when you switch it back over, the first tank will be 3/4 full or something like that because of the return lines on the original tank.
That is certainly one way to do it. The original setup did not use high pressure pumps in the tank, it used low pressure pumps in the tanks feeding a reservior on the frame rail, with a high pressure pump inline. None of the dual tank specific parts are available now, so you'd have to devise your own setup.
The reason for his method of tank switching is because the valve he used won't switch the return to the aux tank. His valve switches between the main and aux tank for feed, but the return goes straight back to the main tank. You've got to make sure that you alway pull the main tank practically empty so that the return doesn't overflow. Then run the aux tank down, then switch back to suck the returned fuel out of the main.
There are valves available that will switch both the supply and return lines. IIRC it switched the pump power too, but it wouldn't be difficult to wire that up if not. It's been talked about in the past in previous dual tank threads. It's the valve used in a later full size dual tank truck. I don't know if you truck uses an in line or in tank high ressure pump. Since you won't have a reservior to feed an inline high pressure pump, you'd be beter off going with in tank high pressure pumps.
Probably the best resource for info on this is the 'yard. Head to the junkyard and find a factory dual tank (those were only 1st gen if I got my info correct 83-87), scope it all out, take pics, etc etc. I know of one in a junkyard near me 6' short bed. The durable parts you could pull from that donor.
I'm certainly not going to say that it isn't possible, but good luck. I've seen several online, but in my 25 years of being a ranger enthusiast and junk yad hunting, I've seen exactly one dual tank Ranger in person. It was not in a "yard", but it is now in my yard. It'll be another yearor two before I start really fixing it, but I had to get it.
If you do find one in the "yard" most likely the only good pieces on it will be the crossmember, straps, and the 13 gallon aux tank itself if not rusted out. I think the tank on mine is plastic so it might have survived, but I haven't verified that. The switching reservior on the frame is a known failure point for the system, IMO not worth buying if you can't check that it works. Most likely, even if the tank is good, the tank sender is going to be rusted up and corroded. Again, replacements aren't availalbe for any of those components.
So, say I obtain a Bronco II tank (idk if it'll fit under my truck because I just got a leveling kit under my truck) and hook up a filler neck and a fuel line and connect a 3 way solenoid valve to the original fuel lines and the BII lines, would that be theoretically possible?
Some people have installed a 23 gallon BII tank in that same area, either as an aux tank or a main when the original was removed. It's got about the same foot print, but much deeper. You'd have to remove the spare tire mount and build a crossmember to hold straps for the front of the tank. Be aware that the BII tank will hang down a few inches below the bottom of the bed. On a stock height truck or lifted truck, the tank will be visible under the bed. Not as much so on a lowered truck due to the angle, but then you've got to worry about ground clearance and stuff in the road.
Full disclosure. In a previous build for a 2/3 lowered 80s Ranger, I was going to be removing the original tank and running the BII tank behind the axle. Went so far as to but the tank and everything to install it. IMO if I were doing it now, or needed to replace the aux tank on the 85, I'd probably measure and try to find an aftermarket cell in the 10-15 gallon range that would tuck up in the hole and not hang down so far. Possibly even measure and have one built for the application.
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