This is just a proof of concept written in C++ to answer this StackOverflow question about Qt and docked windows.
When moved, each window resizes and moves the other via the overridden moveEvent method. To avoid endless looping, the window which is moved by the user inhibits the other window's moveEvent. This is achieved overriding the eventFilter method too, to catch the mouse click that starts the dragging operation. When this happens, a flag in the other window object is set to true, thus making its moveEvent do nothing. When eventFilter catches the left mouse button mouse release (that means the dragging operation is ended), it resets the other window's flag to false.