Just as much as he had wanted to see and touch Stiles, Isaac had wanted this; to see Stiles and Derek together. He knew that Stiles had a bit of problem with self-esteem, when it came to pleasing Derek, but watching his fellow slave swallow around their Master's cock made Isaac question how that could ever be the case. The sight of the boy's lips stretched around the girth was enough to make Isaac moan, on it's own.
Not wanting to give up the view, he slid two fingers into his mouth, slicking them with spit, and teasing at the edge of Stiles' hole. The boy was already opened up a bit from his tongue, and it only took a few passes, before he opened up to Isaac's fingers as well.