How to Connect Multiple Monitors to a MacBook Air
As has been mentioned, you couldn't daisy-chain multiple Thunderbolt displays on pre-2012 Airs. Two possible add-ons that'll allow you to connect an additional external display:


Matrox DualHead2Go (in action hooked up to an 11" Air)
DisplayLink adapter


I've used the latter and they work pretty well for non-intensive applications.