I don't like the idea of duel engines, but if I was forced to build one, I would simply have the smaller engine that is used for cruzing set up as a single speed, with a free wheel, so that engine would only turn the wheels if the engine was turning faster then the wheels, and start that engine when you hit the highway, poping the larger engine out of gear and turning it off.

I had an idea of using a very small engine to run a turbo, or supercharger. Kind of like the small turbos that are made just for FE, except it would be run by a small gas, or electric engine. I don't know how much load would be needed to run an electric one, and if the tradeoff of a larger alternator would be worth it, nor do I have the money to develope it. If it works, it's a $mil. idea.
Patent pending... J/K

The problem is, as someone pointed out, synchronising the two engines.

There have been only one (maybe two) cars in all of history ever made with two engines, at least one of them was a high dollar exotic with two V-8's side by side (and evidently mpg was not the goal here lol).