You can use shape to simulate split bar between windows (set SpecialEffect to 0 - 3D and MousePointer to 9 - Size W E). In MouseDown event set some global flag. Reset this flag in MouseUp event. In MouseMove event, if this flag is set (user try to drag the shape) move shape with the mouse and simultaneously adjust Left property of the control, which is on the right side of the shape. You must also adjust width of both controls, which are from the left and right side of the shape).