Though we don't really seek them, we have had fun doing FMF's but ONLY when there is special chemistry between the 2 women. Since the Mrs isn't fully Bi and likely never will be, this is kind of a tall order, but it has happened.
Though the Mrs enjoys seeing me have fun with other women, I was NOT (by preference) the focus for the FMF. The SF generally is, and my focus is to try my best to satisfy both women in ways that women are not equipped to.
Not that I mind and this is far from a complaint, but it is kind of like extra work for me to do an FMF. I prefer to focus on the girl/s having a good time rather than expecting that I'll be the focus of attention. I suspect that I'm in the minority there as well, but it's probably how we get to do something that everyone else seems to want and can't find.