[xmonad] One status bar process, visible on N screens

Hello, When running unity-2d-panel, the panel appears once on each screen, even though a single process is running. (There is a glitch where the width of the narrower screen determines the width of the panel, so it doesn't span the whole screen width on the wider screen.) At the moment I am spawning two xmobar or dzen2 processes, one for each screen. This isn't really satisfactory: Requiring multiple processes merely to display exactly the same information, and hard-wiring 2 bars, is rather grating. Have you any suggestions on how to configure XMonad so that a single status bar is visible on every visible screen that happens to be connected at any time? Thank you.