Howdy, There's a couple of reasons why two rubbers are/can be used.
1. On drop targets, sometimes the ball goes under the rubber and lodges itself inside the rubbered area...On my 7 target Sky Jump this was a problem. Rather than use two rubbers, I placed a Bally mini-post just behind the existing rubber, in the spot where the ball keeps 'going under'. (This distance was so long on Sky Jump that I wound up using two mini-posts...one behind the '3' and another behind and between the '5' and '6'.) If you place it a couple of millimetres away from the existing rubber, then the normal rubber will still act with bounce and spring. The only difference will be that the ball will no longer go 'under' the rubber.
2. Using 2 rubbers on the flippers and sometimes on 50's style rollover lane guides reduces the 'space', and therefore gives greater average play time for each ball. ie: it makes the game play more 'liberal' or easier.
I hope this is helpful. Mark