PLEASE do not worry! Do not worry! Do not worry! I know it's hard not to want to research but there is too much information on the internet, and not all of it is accurate! I have identical twin girls. When I was pregnant, it was difficult for the doctor to find the separation between the girls because they were sharing a sac... As the doctor monitors you, he will tell you if there is a concern. They will monitor the weight of each baby and as long as one of them does not start to grow a lot faster than the other, there is no cause for concern. Even if that was to happen, this is a manageable situation. At 16 weeks, the difference in weight would have already been noticeable. There was a difference of mere ounces at 12 weeks for another reason with our girls, but the point is that the doctors noticed immediately! My girls were born at healthy weights and are now 9 years old.
If you haven't already looked into it, I am a member of the North Dallas Mothers of Twins Club, a non-profit dedicated to moms of multiples. Our national parent is National Organization of Mothers of Twins Clubs. You should check in your area for such an organization. It is an awesome way to connect with other moms who have gone through any and everything that you may encounter.
You are going to have a great pregnancy and be blessed with beautiful babies! Take care of yourself and rest!
All the best to you!