I use a 3 ring gag out foxhunting. I have used just a single rein, but have found that I like using 2 reins much better.
My horse is very well behaved most of the time and very sensitive to the rein aids (most) of the time. If I used just one rein, either on the 2nd or 3rd ring for the entire hunt, I would have been using too much leverage for him even with just a light feel to the rein.
With two reins, I spend 90% of the hunt with contact only on the snaffle rein, but when we get to the gallops in open fields and others are passing us, I have the 2nd rein to remind him he should listen. Using 2 reins allows me to ride the horse in the moment, well behaved horse gets the snaffle, the freight train galloping on his forehand gets the 2nd rein. As soon as he is behaving, we go back to a snaffle contact.
It requires a lot of attention, but I’m hoping that with time, he will become a purely snaffle ride. Until then, it’s nice to have that extra leverage, but only when I need it.
I would think that roundings on a gag would mute the amount of leverage of a single rein. By how much? I’m not sure. I learned to ride with double reins at 12 years of age, so I don’t find it that difficult to change my contact on each rein or to pick up and be ready to ride at a moments notice.