I can't imagine it would be that much of an improvment. Instead of hauling around 1 heavy high perfomance engines, you now have 2.
I think the hybrid design in general is flawed because of that reason. Sure, a car like a prius gets great mileage, but when you really look at it, it is a small car with 2 small engines. My brothers 1980's golf diesel got close to 60MPG.
If you want an economy car, I think there are cheaper and simplier ways to do it. If you are reading this and don't agree with me, please realize that I know very little about the subject, and I am largely talking out of my ass.
Never the less, I think hybrids are over-rated.