So you mean you want two slides to show at the same time while you are
projecting a presentation or do you want to work on two slides at a time?
For the first -- you can can't. You could fake it by placing images of each
of the slides side by side on another slide.
To work on two slides at a time, the directions vary slightly between PPT
2003 and 2007. It's Window > Cascade in 2003 and in 2007 you can use View >
Cascade.