I've been meaning to get a solution like this for a while and thankfully I saw a demo of the Black Rapid Yeti by B&H at the Kelby seminar last week. An adapted version of the Sport it allows for the mounting of a second camera or the under-arm stabilizing strap (when a second camera is attached the stability strap can be added under the opposite arm). Perfect solution for me since I'm often walking about with 2 cameras. Speaking to another photographer there he said that he does sports with a long lens mounted to the other strap via the tripod mounting arm and can swap lenses on a single camera much faster, but that still leaves you having to put the smaller lens somewhere ... unless both lenses have a tripod mount and the camera is free.
Anyway, I should have it tomorrow ... the day after my D600 goes back to Nikon (figures).