with an entire extra cab its completely do able since you will also need more rocker to add on for the added door length.your going to have two seems either way.if you really wanted to have some fun .you could do a true quad cab.now i don't mean true 4 door but make the extra cab section open in suicide fashion.
if you cut you're cab right before the turn up for the B pillar ,stretch it back the 2 feet or what ever.then take you're the other cab and take the center section out of it.weld that in as your fill from the roof to the frame rails .leave some extra frame rail on the bottom of it to sleeve into the original frame.this will help strengthen it.now this ring of body so to speak gives you a full length door opening from the front to the back as well as a roof opening.from this point you take the original Comanche door and cut the front off of it and section it down to what you need.go to a junk yard snag the rear doors of a 4 door Xj as well as the B pillar.take the B pillar weld it to the front of the rear door.section the front of the door off and weld that to the rear of the Comanche door.this gives you the rear door with the ability to make a working roll down window.put suicide hinges on it and get some latches from say a Ranger or S10 and get them into the B pillar area.next bolt on the front doors of the 2 door.
here is a guy that kind of did it with a 2 door Xj
of course if you started with a Comanche and do it the way i describe(can you tell i have thought about this a bit) you get to keep the square corners of the cab,you get work doors all the way around for easy access to the rear seat or storage,and it stays clean looking if not almost factory.if you don't stretch the frame along with it you don't get enough room before the rear wheel.don't the long bed and short bed have the same wheel base just a different bed length ,longer over hang to the rear.if so the rear tire would be right there at the back of the cab.