LastApp cydia tweak allows you to quickly switch to previous application.
With this extension, you can quickly switch back-and-forth between two applications.
Once installed, you must choose an activation method via the Activator preferences pane in Settings.app.
If you have Backgrounder installed, you can optionally set the current application to background upon switching (unbackground via the normal Backgrounder method). This option can be enabled in the LastApp preferences pane in Settings.app.