Do you have the daygame blueprint? They talk about approaching two sets a few times.
Basically, the gist of it is if they are as hot as eachother, open them both the standard way, with a compliment. Something like you love their energy together, you can tell they're really good friends. However if one is clearly better looking/more appealing than you open the one you like and say to her friend "I hope you don't mind but your friend is like really cute do you mind if I chat her up a bit" or something to that effect, non apologetically, obviously. Mothers are easy, you pretty much do the same thing, but the mother will usually be as flattered as the daughter and actually walk off to let you do your thing.
Practice makes perfect. But yeah defiantly start approaching 2 set if you want to take your game further and let us know how it goes!