I added two dock windows to main window with following statement:

moveDockWindow( dockwindow1, Qt:: DockLeft );
addDockWindow( dockwindow2, Qt:: DockLeft, false );

when main window is large, both of dock windows are on left in one column. However, when main window is small, the two dock windows are automatically put side by side in two columns.

Question: how to keep both dock windows in one column when main window is small?

BTW, in main window, follow properties are set:
setDockWindowsMovable(false );

Thank you,