Open 2 presentations.
1. Click tools>>Options.
2. In the view tab, under the category Show you will see "windows in taskbar"
3. Uncheck windows in taskbar and click ok.
Now you will have two powerpoint presentations in two seperate windows within the PowerPoint application. Click on the Restore Window button in the upper right corner for the slideshow (not the PowerPoint application). You should now see 2 presentation windows within the powerpoint application.
PowerPoint 2007 gives you additional window functionality on the ribbon interface.