Either you find a girl who offers duos with their friends
The way I do it, I pick one of my ATFs whose also bisexual, surprisingly a lot are down, ask her who she likes doing duos with or who she would like to do it with, then go from there + bottle of wine.
I do a duo every year on my birthday (im actually late on this... fuck). I usually sit back in my bed with my ATF and we both shop for girls on my iPad lol.