Twins ? Or yolk sac? Blighted ovum? Vanishing twins?

We went to an ultrasound center yesterday - so not a dr office - and there was what looks like twins in one sac. Which is extremely rare (1 in 50,000) so hard to believe. Could the bottom one be the yolk sac so far away? We got a strong heart beat on top, and maybe a flutter on bottom but couldn't confirm. Has anyone seen this? Roughly 7 weeks 1 day