We also used to play Doublers against a wall with 2 balls, the show offs used 3.
I can still play most of the Doublers routines [only we just called it Two Balls]. Our routine was:
6 of under
6 of over
6 alternating under and over
4 bouncy [where you bounced the ball against the wall, caught the one coming off the wall with other hand ....repeated
3 bouncy [stood astride and bounced the balls through the back of your legs] Can't do this one any more, I think it involved skirts tucked up the leg of your knickers, or short shorts.
2 dropsy to end the routine, just throwing the ball against the wall and letting it bounce into your hand.
I could play 3 balls but couldn't really do any routines with them, nor get up any speed.