Ask the parking people to write something on your windshield when they scan both passesRegardless if they paid for 2 or not, there's absolutely no way you can prove (to average Joe or parking enforcement) that you haven't already used the 2nd pass to get a car in...
I've seen the parking people make people move tents/chairs/bags/etc out of extra spots. Not sure if showing them your pass on your phone (that may or may not be used) will keep them from making you move.

