Well, as the storyline tells us, Yuki and Carol used to fool around in their freshmen days back then. Eventually Carol stopped playing these games but Yuki couldn't accept it and get these dirty thoughts out of her head. And since Yuki and the MC are into their massage sessions and the MC keeps liberating the girl, all Yuki's feelings and memories came flooding back and even with greater force so she keeps trying to persuade Carol. However, Carol's like an impregnable fortress most likely because of the relationship with the MC. Although, the playthrough with the MC rejecting Carol leads to the same with her rejecting Yuki anyway. Does this mean that whether Carol is no longer interested in these lesbian games, or just the dev didn't bother with the development of this aspect? Who knows.
So leading up to your question I can assume such a scenario will be possible in the future, at least if the MC's on the path of both of them (then at least it'll make any sense). As for Yuki as a solo partner, I'm pretty sure that's not going to happen based on what we can see now.
And don't you think if Yuki cheats on us with Carol, would it be really such a cheating?) Just an extra enjoyable twist of the storyline I guess)