The twin-engined car concept has been done a few times, I rmember the twin-engined Audi TT on Top Gear a few years back. My friend subjected me to some max power dvd thing some time last year that had twin-engined Vauxhall Astra/Nova/Calibra/something else that wasn't really worth the effort. They put it in a drag race with a bunch of other boy racer's wetdream cars. If memory serves, they all got spanked by a squillion horsepower Nissan Pulsar.

To conclude, I think the general concensus is that it's more hassle and weight than it's actually worth. Just having a bigger engine in the middle of the car is a better solution afaik.