Unfortunately, once you have started preparing your returns together, there is no way to separate the returns. You would need to create a new return to file separately. 

To start a new return, select “Start a New Return” from under “My Returns” in the top left corner. Once you have started your new return, you can select “View All Returns” from under “My Returns”. Then delete the unwanted return by clicking the small Trash Can next to it or “Delete” under the return’s name.