In a single plane, think of 3 CreateWindowEx() windows side by side. The left one is a normal interface windows, the middle one is the splitter window and the right side one is another normal interface windows. The splitter window is where the control of resizing the left and right windows is performed. By using the mouse location, you can drag the splitter left or right. Within the control of the mouse moving the splitter window, you use MoveWindow() to control all 3 windows.
The left side window has its right side border changed. The splitter window maintains its width and the right side window is moved left to right with its right side border restricted to the right side of the parent window.