I don't know if its proof per se, but...
I know that in an experiment, a group of physicists got a particle to co-locate. Meaning, the *same* particle was present in two places at the same time. They captured an image, and you could literally sit there and "see" "both" particles.
Allthough, I read about this a long time ago and can't remember all the details now. BUT, if you stop and think about it for a second, wouldn't multiple universes be required for this phenomenon to exist?
And, I can't remember how now, but the experimenters were frankly flipping out because they knew it was the same particle, the exact same particle.
So perhaps an experiment which prooves multiple universes would work something along those lines?