I did this very thing before I educated myself. I went to a Petsmart, asked an employee working there if it was okay to put two females together and was told yes it was fine as they don't fight the way males do. Long story short, 24 hours later I had to separate them because one was going after the other one. Thank goodness I was watching them closely.
That taught me a valuable lesson that sadly many petstore employees are not educated when it comes to bettas. And, it was my error not doing my own research first.
Walmart has a sign in their fish area that says females don't fight, are not aggressive and can be put together. I think it is a huge misconception!