Partner with another fuck buddie?
hypothetically:
Would not be able to stop the partner, I guess. Hope the partner would be having safe sex. Would insist on safe sex then myself too. There is no stopping a partner from falling in love, or being attracted. Would it work in a relationship? No idea, would depend on how deep the other relationship would grow, I guess, and on how it would affect the relationship.
I don't see myself capable of having sexual relationships with more than one person at a time. Sex, somehow is connected with bonding for me. And having more than one significant other would blow my mind off. Not enough social skills for that. Would do my brains in. At the moment life is socially demanding enough without a partner.
A friend I used to have was a bigamist. Worked well for years. Two kids with the two women too. The women had no problems with this. Practical reasons was why one of the women told her partner to choose just one family to live with. Both women knew their partner was one who would have flings where ever he went. So, yes, it is possible. For the really socially talented, I guess.