I want to see her suck his dick

Catching the girl's eye you mime a blowjob and raise your eyebrows as you look meaningfully down at the slut with a mouth stuffed full of your cock.

The guy sees this and, in favour of your plan, puts his hand on his girl's shoulder and encourages her to kneel. She doesn't take much coaxing, dropping to her knees with her mouth already wide and eager and almost immediately the two sluts are kneeling back to back each with a cock totally buried in their mouth.

This isn't the kind of thing people ignore and you find yourselves becoming the centre of attention fairly quickly.