I have the Black Rapid double strap and although it is a bit pricey, it does work very well. I have worn it with two 1D bodies (24-70 & 70-200) during triathlon and other sports events and it is excellent. It does pay to take your time to adjust the length of each strap very carefully for comfort and reach.
This is particularly true if you have a larger eg 1D sized body on the left strap - for landscape orientation it is nice and clean but I have found that portrait orientation with the left hand strap can be a bit restrictive as you pull the strap further across your chest with the connection to the camera furthest from the left. I find that I have to lengthen the strap to minimise/avoid this problem which can mean that the camera hangs a bit lower than ideal when not held.
As for moving about loaded with two cameras attached, then as with other comments on this thread, I firmly rest a hand on each body with lenses pointing towards center of my back. This works fine for shorter sprints - if I need to move further, then I move the adjustable kickers up both straps so that the cameras rest more on my chest - then, again, with hands resting on cameras, I can move quite smoothly, comfortably and (for me) quite swiftly as well. Btw, this principle also comes in handy if you find that you're likely to use one camera for a given time and that you need to crouch down/knell without the free camera hitting or dragging on the ground.
just my view......