My first thought needed disassembly, but you're saying that the Dave's part can't be disassembled? I had thought to press a dowel pin into one of the inner shaft ends, and make it locational slip-fit in the other inner end.
I have almost all of the parts, some of them completed, to do a dual case with one shift lever that works like normal. Lever shifts the front range box and the 2-4. An external air cyl shifts the rear range box. I sold the truck it was intended for some years ago and haven't worked on it since. The way that I made it easy to remove the pin that holds the inner shift finger on the shaft was to put an 1/8 NPT hole in the top of the housing that allowed the pin to be driven back out.