8 weeks pregnant with twins but only 1 heartbeat?!

Hi
I am 8 weeks pregnant. Due 5/28
I went for a vaginal ultra sound at six weeks and saw one sac and no heartbeat. Doctor said it was a little too early to hear heart. Went back week 8 and Dr said I was having twins. She said she saw TWO things on the ultra sound but one was evidently smaller and there was only 1 heartbeat. She was very confused as to what she was seeing. She thinks that the second twin is a vanishing twin and will dissolve. However, she is sending me next week to a high def ultra sound in a different office to be sure. Is there a chance that baby2 can survive? Can twins develop at different rates? IE: possibly twin 2 is 1 week behind in development?
Also the month before I did not have a regular period. I had a miscarriage. Perhaps I ovulated twice??
If anyone has any info on this please let me know... the waiting it out is killing me!!!
Thanks so much!!

[babyprayers]

I did IVF and had twins. I had my ultrasound at week 6 and saw 2 sacs, week 7 only 1 heartbeat,and week 8 2 heartbeats. one could have implanted later then the first. hope thats the case.. good luck