The F-82 "Twin Mustang" was conceived during WWII as a long-range fighter escort. The F-82 utilized two extended P-51 fuselages, mounted to a newly-designed center wing and tail. The plane incorporated new prop assemblies and a unique, four-wheel landing gear.
In short, while the plane would be similar to the Mustang, in reality it was an entirely new beast.
The F-82 wasn't ready before WWII ended, but escaped cancellation, and planes entered service in 1948. It was the last piston- powered fighter ordered by the USAF. It flew as an escort for SAC B-29s, and as a night fighter in Korea. It was retired in 1953.
Originally printed by the USAF and North American, this pilot's handbook was de-classified and is here reprinted in book form.