I think it is likely that another sac would have been seen, even if they didn't both yet show a heartbeat.
having said that, was there any reason for the sonographer to look for a second sac? (eg fertility treatment)? I think it would be possible to miss a twin at this stage if the sonographer wasn't very thorough!
They should show up but I have read about people who had an early scan showing 1 and then a 12 week scan showing 2!!
I only had my first scan at 12 weeks and twin 2 didnt show up until she was "checking that there were no more in there" It actually looked as if the screen flickered as they were lay in identical positions lol
Sorry to hijack but I just wondered if u had any idea from ur pregnancy sypmtoms that u were expecting twins or maybe a feeling? Just had a bfp and have feeling am having twin girls????? This is my third pregnancy but they only time I have had an early bfp two days before period due and have been having strong symptoms over the past week! Tell me I am insane ! Previously normally get my bfp after four days late! Good luck with ur pregnancies x