I'm not speaking from experience here, because I haven't tried it, but here's what I think. You're going to need Windows 98 or 2000, because you're going to need something that supports multiple monitors(obviously). I don't know if you can do this with 2 video cards, or if you need a dual-head one(like the newer Matrox cards).

Once you have that, you'll want to stick your debugging window onto the monitor you aren't going to use for graphics, and make sure that when you initialize your graphics stuff(I'm assuming that's you're working under windows here...) make sure to use the proper GUID for the device that uses the monitor you're working with.

Once again, this is simply off the top of my head, but maybe it'll help out a bit

Jonathan