Quoting Mike Meyer
It may take some finagling to get things started properly:
DISPLAY=:0.1 xmonad & # Managing screen 1 DISPLAY=:0.0 xmonad # Managing screen 0, exit to exit X.
It doesn't look like xmonad has a way to specify a non-default config, which would significantly lower the value of this.
The config file is a program written in the general-purpose programming language Haskell, so you have all of Haskell's power at your fingertips. Branch on the value of $DISPLAY or do any other thing that seems sensible. For example: main = do display <- getEnv "DISPLAY" case display of ":0.0" -> xmonad config00 ":0.1" -> xmonad config01 _ -> hPutStrLn stderr "Hey! I was expecting either :0.0 or :0.1 in $DISPLAY." ~d