Horse back riding with double reins is significantly more difficult than riding with a single set of reins and requires more skill the riding with the traditional single reins.
There are two ways to use double reins when horseback riding. The classical way to use double reins when horseback riding is the double bridle. The double bridle is a piece of equipment used on extremely well trained English horses. This bridle actually has two bits in the horse's mouth, with a set of reins attached to each bit. A skilled rider can communicate more sophisticated cues to the horse using double reins and a double bridle.
The other way to use double reins on a horse is using a Pelham bit. The Pelham bit appears on the surface to mimic the double bridle, but it's actually much less sensitive that the double bridle. The Pelham bit is one bit that allows two reins to be attached. Each set of reins is attached in a different area of the bit, and which rein is used determined how severe the action on the horses mouth is. The wider rein is attached to the snaffle ring, and when contact is taken on the snaffle ring of a set of double reins, the pressure on the horse's mouth is relatively gentle. The narrower rein is attached at the end of a shank of varying length. Depending how long the shank of the bit is, the action of the second of the double reins can be relatively mild or more harsh. The Pelham bit with double reins is frequently use in jumper classes, saddleseat classes, and is sometimes seen in Hunter classes. This use of double reins is not allowed in dressage.
Double reins are only appropriate for more advanced riders, although the double reins of the Pelham bit do not require as much skill as the double reins of a double bridle. If you think you are ready for double reins, or your horse may benefit from double reins you should consult your horse trainer.