Do you want to use the presenter screen in a mulit-screen setup but also want to be able to do a demo on your second screen and have it shown on your presentation screen, so everyone can see it?
After you start your presentation, you can always mirror one of your other screens to the presentation screen.
In the screen shot above, you can see a two-screen setup: A presentation and the presenter screen. The presentation is currently on a slide that says "Demo Time", but the feature works everywhere. To start screen mirroring, click on the link "Mirror your screen..." (1).
Now you can select the screen you want to mirror (2) and click the "Start mirroring" button (3). From now on, your audience can see everything that's happening on the screen you just selected.
Minimize the presenter screen (4) and do your live demo. When you are done, switch back to the presenter screen and press the "Stop mirroring" button (5). Now your audience sees your presentation slides again.