It’s an extraordinary sensation, DP is. I’m sort of assuming Clegane will be the bottom of the cookie, so to speak, with Brienne as the filling and Tormund topping the lot, which is good. He’d have the most experience with this, and he’ll be in the trickiest spot. But for Brienne, it will be an overload of sensation. There is an added level of stimulation from feeling both cocks moving in you, and pressing against each other, with you in the middle. If the men set up a slightly alternating rhythm, it’s absolutely breathtaking. It’s also incredibly empowering, because both males are going to be just as caught up in the sensory overload. They’ll be surrounded in tight heat, and feeling something big and hard pressing against them every time they move, or even breathe.
I think you’re right about Clegane being easy to keep in character, and Tormund, too, since he seems a natural as a teacher, or guide, for them both. But Brienne? This is miles outside anything she would ever have dreamed of, and I think she will either let them know (“Move! No, don’t move!”), or be reduced to almost animalistic moans and grunts.
But really, this will be brilliant for her, and enormously fun to read, I suspect!