Windowed DirectDraw applications written for the null or default display driver will work on MultiMon systems, but in applications optimized for MultiMon you will want to create a separate DirectDraw object for each device, using the GUID returned in the enumeration callback. (See Enumerating Devices on MultiMon Systems.)
Avoid setting the cooperative level multiple times on a MultiMon system. If you need to switch from full-screen to normal mode, it is best to create a new DirectDraw object.
It is good practice to release all DirectDraw objects at the same time. If you release only the secondary device or devices, the primary device goes back to its original desktop mode, but only the taskbar is redrawn and the DirectDraw primary surface is still present. You cannot draw to this surface without first releasing the DirectDraw object and then re-creating it.