I've fiddled around with a similar idea... A fork with 130+ spacing, and a "rear" wheel with a slightly longer than normal axle. The special fork goes on the rear bike, the special wheel goes on the front bike, the two mate up and...
You end up with a three-wheeled tandem with two drivetrains. Of course, stoker-induced steering is a very real problem, three wheels isn't horribly efficient, and it looks just plain goofy. All the same, someone would buy it.