OP, please dont use a double bridle without either a decent instructor or someone who knows what they are doing, it is a very precise bit of kit, which fitted and used incorrectly can do a lot of damage.
why do you need or want ot put your horse in a double bridle. If you are ready to put a double on in terms of your riding and the horses level of training then you should have knowledge enough to know what is used. Sorry sounds harsh but doubles in the wrong hands are very harsh. you can remian in a snaffle bridle for higher level dressage now.