It is official: Prism regions don’t work for a tab control inside a tab control. The reason is described here: http://stackoverflow.com/questions/8592802/tabcontrol-within-tabitem.
Simply put, there is only one instance of the outer tab contents, which gets different DataContext when you switch tabs. Thus, there is only one nested tab control. If even you forgo scoped region managers and just use different region names for each tab, it does not matter. The region name on the inner tab control may change, but Prism region manager is not smart enough to change the items of the tab control because of that.
The end result is that if you add view X to the inner tab control on one tab, it will appear on other all other tabs.