Their master forbid them to have sex with one another; he didn't care that they once lived as a normal husband and wife, those days were long behind them, and she was only allowed to be fucked by him. The sissy? Sissies do not fuck. Simple as that.
But their master was not a cruel man, he knew they needed their intimacy with one another, even as they served him. So he allowed them one night per week alone with one another. Just in a manner acceptable to him.
So every Sunday evening at nine, he would bind them to one another, carefully arranging them so sissy's boy clit was trapped between his wife's thighs, pressed against her clit, but tied tight enough that neither could move but a quarter of an inch. Or less.
The position was ingenious, he knew it, too. Tied as tight as they were, neither would be able to move enough to make themselves, or the other, cum. But tied together as they were, sissy clit pressed to girl clit, neither would be able to relax, either.
Instead they would spend all evening, all night, aroused, hot, horny, desperate to cum.
But unable to.