It is not uncommon for one baby's head to not be clearly visible during an NT scan in a twin pregnancy. This can happen due to various factors such as positioning or technical difficulties. In most cases, it does not indicate any serious problem.
The fact that your doctor has scheduled a review after one week suggests that they want to ensure better visibility of the baby's head before making any conclusions. Sometimes, waiting for further development can help improve visibility.
It is important to remember that each pregnancy and ultrasound experience can be different, and it is best to follow your doctor's advice and recommendations. They have the expertise to assess your specific situation accurately.
If there are any concerns about the health or development of either baby, your doctor will discuss them with you and provide appropriate guidance and support.