[brucepick]

I've seen layouts for hybrids where there's one motor, mounted concentric with the engine. It's used as a starter motor and for braking regen and for propulsion when running electric+combustion simultaneously.

I confess to not knowing whether any production hybrids are using this or not. But it makes a lot of sense to me. A relatively simple design in that there's no additional clutching or belt drives etc. needed to make the parts spin. The down side would be that you're spinning both engine and elec. motor/gen whenever either is needed.