I used to fantasize about joining Dude and his ex-girlfriend (when they were together) - mainly about me and him working together to drive her crazy.
(Interestingly enough - a few months later when they were broken up and he and I were together, the opportunity arose and I was not at all interested in pursuing that fantasy IRL anymore; although I would have been up for it at the time when they were together and he and I weren't. if MrS hadn't voiced his objections
I fantasize about MrS being with other women, sometimes I'm "involved" and sometimes I am just watching. MrS doesn't have a girlfriend so these are mainly vague fantasies unless I am picturing girls we were with in the early years of our relationship.
MrS enjoys thinking about me being sexual with other women and has greatly enjoyed the times that he has been invited to watch or participate. (I suspect the same would be true of Dude
Both MrS and Dude seem to enjoy the thought of me with the other one and like seeing the other make me squirm with pleasure. But this may be more about compersion and less about sexual fantasy.
On the other hand I have a strong "exhibitionist" streak (and I think Dude does as well)...but I do find that the sexual experience is "different" if I feel I have an audience. I find that I cast my sexual awareness net farther - and feed my sexual energies off of the other person(s) excitement much more than when it is 1-on-1. I like both - wouldn't want to limit myself to only one or the other.
Just realized that I wandered off from sexual fantasies into the realm of group sex dynamics...obviously some overlap there for me.
Short answer: yes