Awesome! Congrats twins are great.
I just had my ultrasound today and I.am having TWINS! Just wanted to post an update.
The way I see it both options are equally likely. You could either be miscarrying the one, or it could just be developing slower then the other. If I was in your shoes I would go back in two weeks because two weeks is enough time to see a change in the sac. Good luck mana!