Unkl Ian is correct, go talk to the tractor pull guys. In my initial research and digging around I found a few sources that build twins (or triples, quads, quints) for the tractor pull crowd. If you want them side-by-side it gets expensive quick as it requires custom made transfer boxes/cases that's why I chose in-line. I would thing someone in the tractor pull arena would have a used set up for sale that might work and help a ton with the cost. There are a lot of people with twins in the pulling and racing world, but I don't see a lot of info out there one people running them on the street. There are a lot of questions I have about the little things - how to start them, ignition system setups, cooling, balancing, synchronization, ect.
I've attached a couple pics of my in-line coupler. I built it on my lathe and used a dodge-style chain coupler.
Anyone know of someone that custom builds distributor caps? I want to run one distributor that fires two plugs at a time - i.e. to terminals for each firing position.