I had the thought when photographing the steam train with 2 cameras on 2 tripods how handy such a thing would be. So I searched and found Manfrotto make one - see here.

I don't suppose anyone has seen one in action - I can't quite work out how it attaches to the tripod.

It would be great to be able to pan the 2 cameras together on one tripod, and my heavy green tripod should be up to the weight.