Twins to close together??

6w5d. The ultrasound tech said she saw 2 heartbeats and 2 yolks. She said one baby had a heart rate of 135 and the other of 134. BUT she said the babies are really close together so I have to go back in 1 week to make sure that both babies are growing properly and that one baby doesn't "absorb" the other. I'm petrified of the thought of losing one of my babies! Has anyone had anything similar??