There's a technique that worked pretty good for me these last times. I called it "double involvement". I personnaly am fan of direct approaches. So, I go to a 2 set, I express my interest to her friend rather than to my target. I can go full direct: "hello, I find your friend really cute" or role reversal: "your friend seems to like me very much". The point of it is to getting both involved in the conversation, neglecting noone, and having them accepting you.

Note that it can be usefull in a mom/daughter or boyfriend/girlfriend set, where you can not directly hit on your target. Example: "Hey dude, your gf is hot. Wanna share her with me?"