Two Instances of PowerPoint
If you want to edit two PowerPoint presentations side by side you can open them both and choose Windows> Arrange All. If you have a multiple monitor set up it's possible to stretch Powerpoint across the two monitors and then arrange all to have one presentation in each monitor.
The problem is you only get one toolbar and need to flick back and forth between monitors.
Set up a second user account. It MUST have a password. Start Powerpoint as normal THEN start a second instance as the second user!
How? In the Start menu SHIFT right click the PowerPoint icon and choose "Run As" and enter the details for the other user. You need to have Admin priviledges to do this!
You will also find that the second instance saves by default to the second users "My Documents" but this can be changed in Tools > Options > Save tab.