i was watching my 8 week ultrasound and you can see the baby, the umbilical cord and the yolk sac, i would say the shadow on top of your baby is the yolk sac cause is kind of round and the one under the baby the umbilical cord, cause its kind of thin and long.
i was tryng to post my 8 week ultrasound here but i have no idea how, in my ultrasound its much easier to see the yolk sac and umbilical cord, in yours i guess your baby is in a position that makes it hard to see clearly the other stuff, but i think i can see them though.
what you may have seen behind the baby is the yolk sac.
also i dont think they would have missed another baby, they measure the baby and they are supposed to listen to the heartbeat and they would have heard two heartbeats.