Viable twin?

Hello mommas,

I had a FET with 2 5day blastocyst, I went in for a US at 6 weeks and found one sac with yolk sac an flickering heart, I made sure i told the tech to check if there was two and nope just one, then i go in for my 8 weeks US and there’s two sacs but one baby measuring at 7wks5days and the other sac was empty no baby nothing. Has anyone experienced this an had a successful twin pregnancy, or what was your outcome? I’m so happy an excited I have one little bean growing in there, I just don’t know how to feel about the other.