Not sure if you mean like the old Windows method of showing 2 open windows side by side, each taking up 50% of the screen... (sort of like split screen multiplayer console games?)
if so, I know in CS2 you can open both images, then go to the "Window" Drop down menu, go to "Arrange", then select "Tile Vertically" & should show both images in seperate project/canvas windows side by side for comparisons...
not sure if thats what your talking about, but sounds like it?
depending on the size of the images, should also be able to work on one while the other is still visible too...