I use Air Display to convert my iPad in to an extra monitor when I'm not using it. However, I have other iOS devices lying around, too. Is it possible to use Air Display to turn multiple iOS devices in to extra monitors at the same time?
Question: "is it possible to use Air Display with more than one Ipad?”
Answer: "We only support one iPad at a time currently. We've talked about supporting multiple iPads at once and that may be something we support in the future. For now, we're sticking with one iPad. “
Source: Air Display Support Forum
While air display does not allow 2 devices yet, you can install another app called 'Display Pad' and achieve your goal. It's only $2.99 so it really shouldn't be an obstacle. It doesn't have as many features as Air Display, but it gets the job done and by using both apps you can use 2 devices at once on your mac.
My setup is using my macbook Pro with a 24" monitor plugged into it, between those 2 devices I have an ipad in portrait mode running Air Display stacked on some books to elevate it, right below it I have an ipad mini running DisplayPad in landscape mode. All 4 screens work great, I park skype, reminders and itunes into my tablet screens and it's working flawlessly.