Props to Justin Hoyer for trying this. In the finals of the 2012 Winter X Games Snowmobile Best Trick, Hoyer went for the double backflip. Think about this, and consider the physics for a moment. That's a massive sled, trying to get around twice while flying way up in the air. It's mind-boggling.
Hoyer seemed to be psyched out before the jump, making a couple run-ups and backing off each time. He finally committed, but never had the rotation speed. He got around just about one and a half times and his snowmobile hit the ground first ... and you can figure out the rest.
Fairly sure his arm is broken -- a side-effect of a very heavy snowmobile landing on him. Another competitor will also be attempting a double backflip, but after seeing that, I might be rethinking those plans.