Maybe either MacReady or Childs (or both of them) were already copies at the end and they just didn't know it.
Or on the other hand they could have both been "Things" and knew it, but each didn't know if the other was human or another "Thing". Hell, all the humans could have been gone by half way through the film and the rest was just a bunch of "Things" trying to outwit each other. Now that's
They would've had to have been mostly human at least up to the blood test, but after that I don't see much of a problem. Either way, I agree that the almost total mystery of the whole thing is one of the film's highlights.
Also, "I know you guys have been through a lot today, but could one of you help me so I don't spend the whole winter TIED TO THIS FUCKING COUCH."