Why is this useful? If you want to do a detailed, screen-by-screen comparison of two different presentations - say a German original and its Arabic translation - and ensure that everything looks OK with the with animations, effects, text sizing in fields, etc. this can enable more focused, accurate comparisons. Just clicking through the slides in two windows in edit mode might miss something that depends on dynamic elements in the application.
Here's how you do this:
1. Open the first PowerPoint file.
2. Go to Slide Show... Set Up Slide Show... choose Browsed by an individual (window)
3. Click OK. Then press F5. Resize the presentation window as you like and put it wherever you want on one of your screens.