I never trust shop workers to be able to tell the difference. I was once told that the powder blue males were the females of the striped male even though the tank also had silver fish in it.
How large is the tank? Dwarf gouramis are among the more aggressive species of gourami so unless your tank is very large you will probably find that one of the males bullies the other.
Can you take one of them back to the shop and exchange it for a real female?