Yeah. I mean, I'm a bit split on jazz, too. What did they call it in the Commitments, musical mastur...
Oh, right, I see. Ehem. Just play the situation the way it seems to unfold. No point worrying about tests or not. Anyway, who says you have to be passive, and on the receiving end?
Chances are, girl #2 is there for security, and will disappear very quickly if everything is okay. Or she's a backseat driver. Or they do tag-team dating. Or she wants a free meal, too. Look, who knows?
If you're in any doubt, you could say to both of them "Just to get it out in the open: I'm sorry to disappoint you, but I'm not looking for a threesome". That is, if they have that sort of humor...