i've mostly seen it in high level dessage (some one out there can correct that) they have a standard shanked curb bit and a rather narrow snaffle bit so two sets of reins controling two entirely different bits-it is very much a finess thing- i have also seen trianers going from snaffle to a bosal ride with two sets of reins (or visa versa-depending on how you use your bosal) think pelham-but a separate bit for the upper reins. Did that help or hender?
Maggie Attaching picture and this is the explination of it The two bits used on a double bridle: the curb and the bradoon. or go here http://en.wikipedia.org/wiki/Double_bridle for full info
