Let me say at the start that I do not ride dressage, I ride Forward Seat.
I am now riding an absolutely horribly conformed QH that has no hope of ever doing dressage because his neck muscles above the throatlatch are too thick, I swear they project almost and inch beyond his jaws on each side. He had been started as a Western horse (failed barrel racer) and it was obvious he had never had any advanced training.
I asked permission from my riding teacher to introduce the double bridle. She was doubtful it would do any good at all since she was really pleased with the progress I made with this horse with the snaffle. I really was not expecting much either, in fact I expected a melt down from the fact that, for the first time in his life (mid-20s) he had two bits in his mouth.
From the first step in the double bridle this unsuitable horse quieted down. He had been obeying the snaffle bit better but he seemed confused most of the time. When I introduce the double I ride with a sagging curb rein, and occasionally I softly close my fingers on the curb rein with immediate release by softening my fingers.
This horse stopped being confused about bits. Everything became clear to him. I could “hear” him saying “OH, you mean THIS” in the double instead of his usual guessing with “I just do not understand you” with the snaffle bit alone. He has never shown any discomfort in the double (I made good and sure the two bits FIT and were in the correct place in his mouth), we never have had any trouble bridling him with the double, and he acts like he can understand the whole universe better with the two bits in his mouth.
I really like riding in the double bridle. This horse is the fifth one I introduced the double bridle to in my life. One mare I introduced the double to was young and green, green, green, and she also did not “understand” bit action at all, after a few rides with the double she stopped having problems understanding what I meant.
I did not get the rapid understanding about what the bit actions meant with the horses with much better head/neck connection (part Arabs, Arab),they all understood the snaffle bit just fine, but they all got a little bit lighter and did not seem to mind having the two bits in their mouths.
It also helps with horses who get really bored with ring work as it gives them something to think about while they slog around the ring, which means the horses often lighten in response to the bit while at the same time they become a bit more venturesome in their ability to do slightly more expressive gaits.