Can Apple TV AirPlay mirroring use a "secondary" screen/display?
iPad 2 + Air Display. http://avatron.com/apps/air-display

Use Air Display to turn your iPad into a second monitor (totally cool in itself!), and then mirror your iPad to your apple tv. 

OR, if you, like me, only have an iPad 1, then you can use your iPad as the main monitor and mirror your laptop screen. 

Either way, you end up with three screens going, two of them the same.

That's the only way I know. Seems crazy that Apple can't come out with an update that allows the apple tv to be a second, rather than mirrored, screen.