I don't think you can put a TabControl on a TabPage of a bigger TabControl.
Personally, I don't try to make my Forms this complicated. However, if you
want, you can put a SubformControl in a TabPage and use a Form (with a
smaller TabControl) as the SourceObject of the SubformControl.
Another method is to put the small TabControl directly on the Form just like
the bigger TabControl so that the smaller one looks like it is inside a
TabPage of the bigger one. Forn this method, you need to write VBA code to
make the smaller TabControl visible / invisible depending on which TabPage
of the bigger TabControl is selected.