Years ago I had an engine from a Puch/Sears Allstate motorcycle a neighboring farmer gave me to tinker with. It was a "twingle," or split single two-stroke. It had two upright parallel cylinder bores in the same finned casting, one in front of the other. The connecting rods were articulated together kind of like a Harley-Davidson, and both bores shared the same combustion chamber in the head. It had piston-port induction on the front piston skirt as I recall, and the exhaust ports were also in the front cylinder. Can't remember if it had transfer ports in one or both. But unfortunately the cases were super-corroded because it had been sitting in the middle of a big fertilizer spill in the guy's barn. So it was useful for educational purposes only. I'd have loved to hear it run. Can't remember what I did with it now.