I had to look it up on Wikipedia:
Counter-rotation was achieved with the use of "handed" engines, which meant that the crankshaft of each engine turned in the opposite direction of its counterpart. The V-12 engines only required that the spark plug firing order be changed in order for the direction of the crank shaft to be reversed, according to the General Motors Allison V1710 Service School Handbook.
Lockheed P-38 Lightning - Wikipedia, the free encyclopedia
sorry for the thread drift.
regards
Howie