Right now I´m thinking about a solution somewhat related to my displacement reduction project. -Instead of the standard solution with a 4-in-line two identical 2-cyl engines on separate gearboxex side by side! This would offcourse be heavier than with a common gearbox and separate cluthces but by having two separate systems I get a unique advantage: reliability! - If one engine or garbox breaks down I can still continue my trip. When extra power is needed both engines will run and also give better traction than with a differential between the wheels. While in eco-mode I can alternate between the two engines to wear them equal or just wear one out before the other.
Two gearboxes for half the load each should be about the same weight as a single transmission minus the weight for the differential. Perhaps more friction losses are added and definitiely more complexity to linkage and engine control. Offset traction while using one engine only may be a obvious problem. There must be an rpm limiter while using both engines in case one tire suddenly lose it´s traction while accelerating.
I don´t think this is a concept for mass production but for a DIY:er it may be interesting. It may also be suitable for "scrapheap challenge enthusiasts" like myself.
Now I just have to check if this crazy concept can fit into an old Saab 96! A unique feature with these cars are their freewheel. The Saab´s longitudinal engine/transaxle placement may be just perfect as long as there is enough room between the wheels. (these cars are quite narrow)