Hi and thanks for your post,
Two females from the same litter should be able to live happily together provided you assert one as dominate over the other and if possible have them both desexed. To assert one as dominate feed it first, let it into the car first, let it have first access to you when yo get home and let it have first access to new toys. Making one dog dominant in this way should stop any aggression that could occur if the dogs were unsure of their position in the pack and felt the need to be aggressive to sort it out. Remember however that you are the overall leader of the pack (alpha dog) so you have the ultimate control over food toys and attention.
Hope this helps if you have any questions as your puppies grow don't hesitate to ask.