Can a sister's father and the imam be the two witnesses when she remarries?

A sister who was divorced is remarrying. Her father is present but since she was married once already she does not need a Wali/Wakeel. Can her father and the Imam be the 2 witnesses if there is no one else present to be witnesses to the marriage? The Nikkah was done with the Imam and her father as the 2 witnesses. Is the Nikkah valid?