I happened across this video about some fanciful engine designs and thought I’d share:
Interesting! Here’s a bit smaller one, only two crankshafts:
Here’s one way to achieve the uniflow scavenging of an opposed piston two stroke engine while only using one crankshaft.
I actually was able to take one of these out on the road for a short spin. Lots of vibration, and it wasn’t exactly a rocket ship. But these did have a reputation for getting good gas mileage.