I want to design a UI for tablets (Android & iOS) with Xamarin.Forms where I want to have multiple ContentPages
As far as I know you can't put multiple ContentPage items on the screen, at least not within the out-of-the-box Xamarin.Forms.
You can, however, put multiple ContentView items on the screen and you can reuse the ContentView items in stand-alone pages as well.
Essentially you can describe what you'd want to see on a single page as a ContentView and then make a page that just includes that
PhotoItem.xaml:
PhotoBucket.xaml:
... on list item tap do Navigation.PushAsync(new PhotoItemPage{BindingContext = itemTapped} );
// Add two columns
... on list item tap do PhotoItem.BindingContext = itemTapped
PhotoItemPage.xaml: