Although many methods exist to move pages between PDF files, by far the easiest is by using document Thumbnails.
Setup your document so you can see both PDF documents in the same window (Window>Tile>Horizontal – see Figure 1.1), then make sure that the Thumbnails are showing for both files (Window>Show/Hide Thumbnails).
From the SOURCE file, highlight the thumbnails/pages you want to move or copy to the other PDF and DRAG these to the other PDF into the Thumbnail window (see Figure 1.2). As you are moving you will notice the mouse icon will show a drag box and a Plus sign, this indicates the you are COPYing the pages, if you want to MOVE the pages, hold down the control key (whilst dragging) to toggle the behaviour (the plus sign will change to a minus sign).
Note: You are not restricted to just two files, open as many as you can fit on screen and move/copy as you see fit. Also remember that Bookmarks are not transferred across.