I was wondering this myself. My wife rented an HD movie through our Apple TV and I wanted to watch it on my other Apple TV that's hooked up to a different TV with better sound.
Since she rented it from the Apple TV, and since it's in HD, it doesn't show up at all in iTunes so I'm not sure if there's a way. Maybe it's different if you rent directly through iTunes through your computer--then you can throw it on multiple devices--but that doesn't work if you want to rent HD.