It was the F-82 Twin Mustang, developed as a long range escort towards the end of WWII. It didn’t see action in that war, but did go operational, and IIRC, scored the first air-to-air kill in Korea.
Found this on Wikipedia though:
Prototype XP-82s, P-82Bs and P-82Es retained both fully equipped cockpits so that pilots could fly the aircraft from either position, alternating control on long flights, while later night fighter versions kept the cockpit on the left side only, placing the radar operator in the right position.
Very interesting!