Extend Tab Control

You typically cannot have 2 controls in the same coordinate position. Are you asking for the tab to have more than one line of tabs - the MULTILINE property - If the tabs don't all fit on one line, this style allows them to occupy as many lines as needed. If this style is not used, the system adds a scroll bar so the user can scroll to the hidden tabs.

It occurred to me you may be asking if one of the pages within the tab control can contain a tab control. If that is what you are attempting, then no, you cannot have a tab control within a tab control.

We use tab controls that occupy the same space, we use the send to back(ctrl+B)/send to front(ctrl+F) to edit them. As long as all the pages across the tab controls have different names and visible variables you can do it.   

I tried all sorts of tricks to get it to work, but no, you can't have another tab control on top of an existing tab.

So is set up a frame with push buttons along the top, which works.

Here is an example that displays a tab-control SCRAA30-AA30REITER2 on top of the first tab of another tab-control SCRAA30-AA30REITER:

            03       SCRAA30-AA30REITER2
                     TAB-CONTROL       LINE 10.1 COL 03
                                       SIZE  R02-REITERSIZE  CELLS
                                       LINES R02-REITERLINES CELLS
                                       VALUE R02-AKTUELL
                                       ID=WORK-ID-REITER2
                                       VISIBLE WK-VISIB-SEITE-01.

            03       SCRAA30-AA30REITER
                     TAB-CONTROL       LINE 01 COL 02
                                       SIZE  WK-REITERSIZE CELLS
                                       LINES WK-REITERLINES CELLS
                                       ID=WORK-ID-REITER1
                                       VALUE R01-AKTUELL.          

Please note that the "little" tab-control has to be defined before the big tab-control.