I can almost guarantee you it wasn't anyone at petco responsible. I work at a petco a few days a week and although im the only one maintaining the bettas, no one is cruel like that...........EXCEPT customers. All the time i notice that sick people have put two in one cup. Besides that, bettas come in tiny little bags. The only time there are two together is two males occasionally, and thats the fault of the wholesaler/distributor. And they are usually fine because there is barely enough room for one to move.
But alas, i really doubt anyone at the store did it. It had to be some sick idiot...