There was only one Chain Lightning prototype built. I'm almost 100% certain that it was scrapped. Pity.

There are a few Twin Mustangs out there in museums. I know of one on the parade ground at Lackland AFB, TX. It needs to be moved indoors. There's another, the "Betty Joe", which set a long distance record after WWII at the Air Force Museum. The "Commerative Air Force" (formerly "Confederate Air Force") had the last airworthy Twin Mustang until it was damaged in a landing accident several years ago. It's now on display at their museum. I heard last week that they're trying to restore it but that's a very slow process. Virtually all of the parts have to be custom made.

I've actually toyed with the idea of building a standoff replica of the Twin Mustang by joining a couple KR-1 fuselages on a common wing. I'd use an inline auto engine conversion (like a turbocharged 3-cylinder Suzuki engine) to keep the cowling looking right. The afterdeck on the KR-1 is non-structural and some have been built to look like mini Mustangs. However, there are some severe center of gravity (lateral) issues with the idea, especially when flying it solo. Still, it'd be a hoot to taxi up at Oshkosh in one of them.