are they both definately female? to tell the difference you look at the underneath fin, if it's quite long and thin then they are male, you may have to wait to see this fin as they keep it up most of the time. If its a more triangular normal shape then they are female. Sorry I'm not that good at explaining!
I had a pregnant female, which I seperated from the other tankmates so that she could have the babies in peace. This meant that I ended up with lots of babies, but I gave them to a local pet shop.
hope your platies are ok, and you find out if they are pregnant soon