I don’t necessarily agree with you about elite 400m guys being able to do it. Some probably never run at any speed for more than 400m in practices. It would be pretty tough for them.
But, for any poster here saying that an elite 800 guy couldn’t do it is crazy. You have obviously never been a middle distance runner. If you have he endurance to properly run an 800, then running a 400 3-4 seconds slower than your 400 speed, and only about 1.0-1.5 seconds faster than your 800m speed is a breeze when you get ample rest (more than 4min would be plenty).