2) Add a second "layers" menu. (not this is not possible from the Image's "Window>Dockable Dialogs" menu).
3) Drag resulting layer dialog tab outside.
4) Disable Auto Image switch for the dock.
5) Now use the two dialogs like you would use a split pane window manager.
Tip: You can use the scroll wheel to quickly switch images using the image selector at the top of the dock window.