The following tip comes from the Apple Pro Training Series: Aperture 2 book published by Peachpit Press.
Once the displays are attached, you’re ready to configure Aperture for multi-screen viewing. When two displays are connected to your Mac, Aperture considers the main display—the one with the menu bar—to be the primary Viewer. The other display is the secondary Viewer. By default, the primary Viewer displays the Aperture application.
Let’s specify the function of the secondary Viewer.