Set up a good long run for them. If you can have a cable stretch from one end of the other yard to the other, then have chain for each that can slide along that, it should give them running room, but still be secure. I'd only put them out there when you are home, of course, for the sake of tangles, etc., but I am guessing they'd get used to it pretty quickly.

I don't know if I've described it well, but it's sort of like how a chairlift works, understand?