I am an engineer, but mechanical is not my forte. For the S2000 motor, there is little that could be welded together. It would be feasible to build a completely custom block based on the dimensions of "2 motors welded together", but it would be very costly in both machining and engineering. You'd also have to mirror one of the heads, so that the intake and exhaust would be on the correct side of the motor.

On a related note, I saw a shop's website that had built a V8 Hayabusa engine. I'll see if I can dig up a link.