Possible VTS

Hi!

We have been trying to get pregnant since last July. Went for first ultrasound at what we thought was about 5 weeks. Saw one yolk sak. Went back today Dr. said looks like there was/is two? Could only find one heart beat, but too small to hear. Measuring small and at about 6w6d. Anyone have any experience with twins and or vanishing twin syndrome? Did you see both heart beats the first time? Super nervous over here. Dr. Didn’t have much answers other than anything could happen, 1,2 or possible lose both. Any responses are appreciated. Thanks!