Two babies, one heartbeat

I had my first ultrasound today where I found out I was only 9 weeks along instead of 11 like we thought. Also, they saw another yolk sac and what looked like a smaller fetus but couldn't hear a heartbeat. I know it's not unheard of for one twin to stop growing and not survive but I'm praying it was just hard to detect and it'll keep growing to be strong and healthy. I go back in two weeks for another ultrasound and we will know for sure what baby #2 is going to do. Has this happened to anyone else? And did your baby end up healthy?