I fear you will only get guesses. Balloon bodies are a human chosen spinal deformity, and it affects the fish in many ways. That can include a lot of the things we look at to take an educated guess at the sex of rams, never an easy one to sex unless of a certain size. The store clerk can make as good a guess as you can. He has a 50% chance of being right about each fish.
Most buyers of balloon fish aren't planning to breed them, so 2 females would be fine as well. 2 males will fight, but may not be very able to.