]Disclaimer: this does not support rotation.
PullOverPro introduces a small tab on the side of the screen which allows you to pin an app and pull it over the current app.
There is also the QuickSwitch menu: by holding the grabber for half a second, a menu will popup.
You can drag your finger up and down to select what app you'd like to pin.
These apps can either be preselected by selecting "favorites" in PullOverPro's settings, or your most recent apps, pulled from the app switcher.