There's no support for separate screen DMD as yet. The best you can do is use something like Actual Multiple Monitors to mirror the DMD to a second display. It does work, but depending on which OS you're running there are some tricks involved and it may not play nicely with PinballX.
Zen's added a movable DMD to PinballFX2 and it's amazing how much of a difference to makes to gameplay.
