That is pretty impressive and really neat.
OK I think you need to make a special connecting rod set for it, to share the same crankpin on the engine. Probably a split rod setup like the Harley Davidsons use. You could also make a master and slave rod like they do on radial engines, but only for two cylinders. A master slave rod would have more bearing surface on the big end to support the combustion process better than a split rod like the HD's use as the crankpin is only so long. Then you have to machine the two crankcases so they can bolt together into a unit, One case has threaded holes and the other has the screws that go through to the one with the threaded holes. That is probably about it, except for setting the timing of course, but each engine would set its cam timing like normal for each half.