Twins sharing the same sack..

I'm 8 weeks pregnant with twins. It's my first pregnancy and I'm in shock. I've been going to the ER for the past two weeks for very bad back pains and discharge. I've been told by the DR's that since they are in the same sack and share the same yoke it's a high risk pregnancy. I was told one was a lot bigger than the other (as if they were a weeks apart). Is there anything I can do to get them to be similar size? I'm so scared and I tend to panic. They said if I feel any pain, get fever, or throw up more than the usual to go back to the ER.. Does anyone have these symptoms? Is this normal with TWINS?  I don't know what to expect. I'm just so nervous and scared.