According to the Android SDK:
To split the action bar, add android:uiOptions="splitActionBarWhenNarrow" to each activity in your manifest file that you want to have a split action bar.
How to do this in Xamarin Forms?
I'm able to set this atrribute for the entire application in my manifest like this:
<application android:uiOptions="splitActionBarWhenNarrow" />
But I have no idea how to do it for just one activity since it is abstracted away by NavigationPage.