Your "double ring" snaffle is called a Wilson bit and it is a driving bit.
"The Wilson snaffle has four loose rings — two attached to each end of the mouthpiece, with two additional "inside" rings. The cheek pieces of the bridle are fitted to the inside rings, while the reins are fitted to the outside two. The bit works with a squeezing action on the cheeks rather than on the corners of the mouth. The Wilson can give superb control and "turning power" for a hard-pulling horse, while not being overly severe. " borrowed from a House and Hound article in 2008.
I do see them occasionally at driving competitions.
Last edited by pegasusmom; Jan. 25, 2013 at 06:08 AM.
Reason: to add information on action