I'm going to dance with my ex in front of my girlfriend
You have to be kidding, how is your ex relevant here?
I want my ex to see me with my girlfriend
Dancing with whomever you please seems to make you feel powerful. Your current girlfriend will be hurt that you are in such close range with her that way. Decide to not do it, otherwise you will feel the fury of a woman scorned.
It would be fun to see her reaction.
I think it's best to reconsider the situation, have you thought of how either of them would feel being played like this? It can be very hurtful and insensitive to them, so why not find another way to have fun like planning a date?