alright your talking to a girl with a boy friend
Chappo: I hope your boyfriend doesn't get mad that you're talking to me.
HB: no it's no big deal i do what i want.
Chappo: you know, we should double sometime!
Here there are two things she might say.

a) oh do you have a girlfriend?
b) ya we really should

a)
HB:oh do you have a girlfriend?
Chappo: no i don't. so do you think your boyfriend can find a date so he can come with us and not feel awkward, otherwise it'll just be you and me.

b)
HB: ya we really should
Chappo: alright so it'll be me and you, and your boyfriend can bring some girl that he finds.

after that exchange she'll be kinda confused
HB: wait what?
Chappo: what? you thought i was gonna let you go with your boyfriend? you spend way too much time with him as it is.

after this she could go a number of different ways so just wing it from what she says. it might not work but it's a step in the right direction.
only been field tested once but it's a winner and winners do what they want. try it out and let me know how it goes.