Look up Tommy Ivo. He used to build and run twin and quad engine v8 drag cars. I have seen some good write ups on his cars, but I was very young at the time and they were in old books. I remember some of the early mistakes being mentioned, and how things were worked out.

I used to see one of the quad cars often enough, on a trailer in front of his house.

I would think that a bellhousing on the front engine with an adapter plate for the rear engine and a stubby driveshaft in between with 2 u joints would be the direction I would head. Clock them however you want and leave each engine with its own starter. Use two group 31 batteries in parallel and use one alternator.