I've thought a lot about FMF doubles, and there are a lot of varieties. I thought I'd share OldYeller's Doubles Classification with you. I assume that a two girl bj is a part of any doubles.
Oh - G1 is the girl being penetrated, G2 is the one not being penetrated at that time.
Straight Doubles
1. Tag Team. Girls take turns being penetrated. G2 keeps it wet while \waiting ;-))
2. 2 on Guy Doubles. While G1 is being penetrated, G2 offers additional stimulation to the guy, such as ball play, face sitting, or titties in the face.
Bi Doubles
3, 2 on Gal Doubles. In this version, G2 stimulates G1 while she is being penetrated. For example, RCG+DATY. Also, G1 pleases G2, such as doggy/DATY.
4. DP doubles. G2 wears a strapon so that the guy and her can Double Penetrate G1.
Excuse me - I have to clean up my keyboard after thinking about this...