If you don't get a double-ended arrow and your browser takes up the whole screen, it is probably maximised - you will see the following in the top right corner:Open the two pages up and reduce them down by placing your cursor on the edge of of your browser page so it becomes a double ended arrow.
Left click and hold then drag the edge until you can see both screens
Click on the middle box (showing 2 pages overlapped) to switch to re-sizeable mode. If the middle box only shows one page/square then you are already in resize mode and you should be able to 'grab' any edge of the window (hover over it until a double ended arrow appears where the mouse cursor is) then click and hold the mouse button down while dragging the edge to any size/shape you want it