How to display "Back button" on NavigationPage even when I am on the first page

Hi,

I have a NavigationPage (lets call it SubNavigator), which I push onto another NavigationPage (lets call it MainNavigator) using PushModalAsync.

That works. I can push into SubNavigator and navigator back and forward in it.

Now, I would like the "back" button in the SubNavigator to also appear, when I am on its first page. When it is pressed, I want to execute PopModalAsync on MainNavigator.

I am aware, that I could get a similar effect by simply not using a SubNavigator, and pushing my pages into MainNavigator.
But doing it the way I describe makes conceptual more sense. The pages I push into SubNavigator are a subtask for the user.

Can this be done?
Would you do it completly differently?

Thanks!
Nathan