I have been pregnant 8 times now so I have had dozens of ultrasounds done. It looks to me like a single sac with a single baby and the umbilical cord. If you could post a better picture we might be able to see better what you are talking about.
Can you try to post another pic that does not have the shadow. It still looks like one clearly to me but not sure if there is something you see behind the square that we cannot see.
Looks like a shodow to me,i would say just the one baby..