What do you think of me bringing  another girl with my girlfriend and me to prom?
That's crazy. Why would you want to bring two girls to prom?
It makes no sense to spend money on one girl for prom when there are other females in the world.
While you will save on expenses by taking two girls to the same party instead of one girl to one party and then one girl to another party, it is weird to do so. People will think you are weird.
I don't care what they think. Everyone has a right to enjoy life and to do what makes them happy.
How about your girlfriend or the other girl? What do they think? They might feel weird about sharing a date since it is unusual for prom.