If you build one good car out of two junk cars what vin do you use?

That depends on which state you are in and which frame and body you are using. If the body and frame are from same car, then there is no problem except in some states,Kansas, for instance, depending on the age of the car, inspects all the body part #,s for a match. If you have replaced any fenders, hood, doors, or trunk lid, then you might need a bill of sale or proof of ownership for those parts. Best to contact your state hi-way patrol inspection office or the DMV. Hope this helps.