My ultrasound showed 2 sacks but....?

It does look like 2 gestational sacs just one is empty. The other looks like it has a yolk sac and fetal pole. Some pregnancies start off with twins but the 2nd one didn't finish developing. Or, it's possible the second embryo is just unable to be seen at this time. Twins can grow at different rates and with you this early, it's possible ya'll just caught it in between. Good luck.