It was redesignated the F-82 Twin Mustang with the Air Force was formed. A few hundred of them were made (273 according to one source). It was designed to allow for two pilots for very long range escort missions in the Pacific, but WWII ended before it entered service. The F-82 does hold the distinction of scoring the first American aerial victory in the Korean War.

I only know of three in existance but there might be some more in museums.

The Commerative Air Force (formerly Confederate Air Force) has one. It's currently in their museum. It was the last airworthy Twin Mustang until it was damaged in an accident back in 1987. I've heard they're trying to restore it to flying status but it's slow going because all of the damaged parts have to be made from scratch. According to this source, the biggest problem was the damage to the propellor hubs. Apparently, there are no replacements available (they were unique to the F-82) and it may not be possible to build new ones. Other sources say that they do hope to have it flying again.

The F-82 "Betty Joe" is on display at the Air Force Museum at Wright-Patterson AFB, Dayton, OH.

The last time I was on Lackland AFB, TX (1985), there was a Twin Mustang on static display on the parade ground. According to this reference, it's still there. I'd love to see it brought indoors out of the elements.