There have been many twin engined cars over the years, some worked a lot better than others it seemed.
The first Dubsport twin engine VW (the blue Rallye) used a 2.8 and a 2.9 VR6 engine. As Will stated, the engines will match themselves to each other.
I remember reading before an interview with the owner of the twin VR6 turbo engined Mk3 Golf who tried describing it as imagine a big guy and a little guy pushing a broken down car. One is probably doing more work than the other, but it still works better than one.
With regard to laws; I had a rear engined Fiesta that I was looking to put two engines in as I've always been fascinated by them, still seen no law stating it isn't allowed, as the forum above seems to confirm.