Possible to have twins if one sac smaller than other without yolk present at about 4-5 weeks of pregnancy?

I went to the doctor and believe I am 4-5 weeks pregnant.  They did an ultrasound and found two sacs.  One had a yolk (but they barely found it at the last minute).  The other sac is significantly smaller and they could not see a sac.  The doctor has interpreted that this was probably a twin pregnancy but the second baby did not make it.  Is it possible for the second baby to make it?  Is it possible this early on the yolk would not show on the second sac?  I am hoping the second sac will develop although my doctor has not given me much hope.  I am not sure if it is normal for a twin pregnancy to have two different sizes of sacs.  Has anyone had any experience with this?

I go back into the doctor for another ultrasound.  The doctor has offered another ultrasound in one week or in two.  I was going to opt for two hoping it would be further along to give us more definitive answers at this time. Is it better to wait?

Thank you so much in advance!  Trying to stay optimistic and positive so I don't impact our healthy blessing in a negative way.  But can't help thinking about the possibility of being blessed with two babies!