I'm pretty confused by this assignment, but I think I understand the basics.
First off you do not need to generate a second class for a second customer or gas pump. You can reuse the first class over again (with a 2nd variable type).
I'm also not sure why you have doing a random number for simulation of when the pump is open or closed. It seems like what would happen is you would have a random number generator that would tell you when a customer is appearing. You would then do a check if any pumps are open (you can have X number of pumps by using an array or a linked list). For example:
GasPump gasPumps = new GasPump[numberOfPumps];
for(int i = 0; i < numberOfPumps; i++)
gasPumps[i] = new GasPump();
Then you can use as many gas pumps as you like.
I think that fixes the problem with using multiple gas pumps.
The next part depends on the assignment. But it should be as simple checking each placement in the array (this is horrible for performance, but whatever!) if one is empty. And if you reach numberOfPumps then you know it's full.
Hope that makes sense.