Adults must have their own account. Create an account and invite the other adult to create a group.
One adult account can have one or both children, doesn't really matter. Once everyone has signed their application, you can submit the application as a group/family.
More info here...
Yes, parents that are applying with minors can apply online together.
The adults need their own account. You can invite the other adult to join you and apply as a family. One of the adults must create the minor application.