Carrying twins is a little riskier than carrying a singleton. Because of this, you will be monitored more closely and receive more appointments and US's than those women carrying only one baby.
Congratulations!
I think the only real risk would be if they share a placenta. I don't have any experience with twins but I have read that if they share a placenta there is a small risk of twin to twin transfusion syndrome.
I thought twins in one sack means there identical twins? Did the doc say there was anything to be worried about.