Unfortunately this is a very common misconception; Female bettas can be just as agressive, if not more so in certain cases, as males. I'm so sorry you were miss-led and it ended up causing one of your new fishy friends harm; its such a shame pet store employees aren't educated properly....
Anywho....welcome to the forum and wonderful world of Betta keeping :) Though you may have had a bit of a rocky start it seems, at least it was just minimal damage that should heal with time and clean, warm water. ;)
Now, a little info on females....
It is true that females can be kept together in some cases, they are called Female Betta Sororities
and involve groups of 4-5+ females in a very heavily planted/decorated tank of 10+ gallons; this is not recommended for beginners because even when done perfectly it is still very risky for a variety of reasons.
HOWEVER, you most certainly can keep both your girls in a divided tank together without any problems. :)
I hope this helps and answers your question....if you have any further questions, please feel free to ask! Also, take a look around the site and read over some of the stickys in the Betta care section; theres LOTS of helpful info there ;)