I have one of the two-motor jobs. It ran fabulously for years until recently when it started doing a little herky jerky dance down the track. After chatting with some guys in the club, doing a little online research, and examining it myself, I concluded it was indeed one motor wearing out and affecting the overall performance.
That said, I would love to do something with these little power trucks independantly. On30 is one option, but I don't think I'm ready for that just yet. Another option, which I think I've seen somewhere else, is to yank out one motor and leave the good one behind.
Then, with a little kitbashing, open up the doors on the non-motorized end and install a dummy engine. That yellow motor flatcar load, originally ERTL now Life-like, I think, may be a good substitute. Someone more knowledgeable about these things may know better what to use. OR, replace the motor with a speaker and sound decoder.
As long as the little guy is running on fairly level track and not expected to pull much (as in a private industry site - a place these little critters were commonly found) then it should be just fine. Paint it a gaudy yellow, green, blue or even orange, and fade it heavily, throw in a dose of rust and grime appropriate to the setting, and voila! New life for an old critter.