blow him out ASAP. i don't know how to do this, as i have never encountered this situation, but time is not on your side here. also, he's entering the group and you're already in it, which gives you the advantage at the point in time at which the other PUA is entering. you have to have a stronger frame than him and read up on some tactics to blow him out (again, i don't know exactly what these are so i can't help on this angle).
or you could just look at him and say "this is my set" (PUA lingo that hopefully nobody else would understand) and with any luck he would get the hint and leave and the set would have no clue what you were talking about. idk if that would actually work, i suppose it would be a coin toss.
or, a 3rd suggestion, you could switch targets, and with him picking up one, it would then make it easier to pick up the other as he's probably isolating that one girl, diverting the other's focus straight onto you and
hand-delivering you an automatic isolation without you having to do any work.
