Possible Twins. Help

Key

Long story short.. My last menstral period was 4/14. I went to my OB on June 1st and he says I’m only measuring about 5 weeks no fetal heartbeat. FYI - I should have been measuring 7 weeks. Today I went to emergency room. Ultrasound shows two gestational sacs but one heartbeat. Sac a measuring 6 weeks 3 days and sac b measures 5 weeks. Can one twin develop faster than the other