The short answer is yes, the longer answer is more complex.
I use and have used two monitors with windows XP for years and love it.
I'll assume that you already have the two monitors working. Let's start with the basics, restore down your plan window and then restore down the ArchiCAD window. Notice that the plan window cannot go out of the ArchiCAD window and that any fixed palettes are cropped by the ArchiCAD window. Floating pallettes are free to float outside of the ArchiCAD window. This behaviour is not limited to ArchiCAD but common to all of Windows XP Applications.
Therefore to achieve what you are asking you will need to stretch the ArchiCAD window across both monitors, this is best done with two monitors of the same size and set to the same resolution to ensure not to crop any of the window area.
I have two different monitors and find it beneficial to place my "maxed out" info box and navigator on the second monitor as I can only work in one window at a time but I understand why you would want to do what you ask as I have tried it myself.
I think that's about all I can think of for now.