1-"twisted split flipper" person A stands on the left but flips the right flipper with his left hand, person b stands on the right but flips the left flipper with his right hand.
2- Clap between flips - ( never tried this and it might get old and hard to ref quick)
3- Timed game- only 30 seconds per ball, if you lose the ball before 30 seconds tough- after 30 seconds you have to stop flipping
4- Objective based game on a machine - like a pin golf objective - ( quickest time to start move your car on Creature would be an example) , We did that once with number of loops in Dr. Who and it was silly but fun.
Anyway just a few quick thoughts