Recently I tried creating a tabbed admin interface in Views - similar to the Content view where 3 tabs exist: Content, Commments, Media. It took a bit of futzing and reading, but ultimately found this:

1. create page with path "parent" and with content that you would normally have for default tab, don't set up menu settings
2. create page with path "parent/child1" and with content that you would normally have for default tab (SAME as "parent"), set it to be default tab and tell views Parent menu item: Already exists
3. create page with path "parent/child2" and with content you want, set it to be menu