I had my first ultrasound yesterday at 6w,3d and found out we have twins also. They are fraternal. Baby A is measuring 6w, 1d with a strong heart beat and baby B is only 5w, 2d with no detectable heart beat as of yet. We go back for a repeat ultrasound and I'm praying everything will be fine. The tech is warning us about the vanishing twin syndrome. Be thankful you heard two heart beats that's a great sign as I have heard the risk go down considerably after that. Good luck to you.