[Flieger]

The 917 crankshaft has the same number of throws and position of throws as a 911 crank. Each throw was wider to fit two rods though. So I am really not sure why you would want to go through all the trouble to put two 911 engines together. I think you'd still have torsional vibration problems whether you took the drive from the center like a 917 or from the rear like a 911.

I'd just use one 911 engine. To put the two together you'd need to make an adapter to hook the flywheel end of one crank to the front end (fan end) of another. Or else convert one to turn in the opposite direction and then mate them flywheel to flywheel, then have a gear setup or chain setup to a layshaft going under the engines to a tiny clutch and a gearbox like the real 917.