Put the second tab control inside a tab panel of the first tab control.
Select the 2nd tab control while in layout mode and copy it to the clipboard.
Double click the first tab control to open Tab Control Setup... and add one more tab panel.
Paste the copied tab control into the tab panel. You'll need to resize it to be smaller than the first tab control so that it fits inside the tab panel of the first tab control. Drag the first tab control slightly to make sure that the 2nd tab control moves with it. This shows that it is properly "owned" by the tab panel into which you pasted it.
If it doesn't move with the first tab control, drag it away from the first tab panel, release your mouse button and drag it back to the tab panel that should enclose it and make sure that it is fully inside the borders of the enclosing tab panel. Release your mouse button and test again to make sure that it moves with the enclosing tab control.