Usually I connect my MacBook Air 11" to a secondary display via the Thunderbolt port.
What do I need to connect it to two external monitors?
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:
I've used the latter and they work pretty well for non-intensive applications.
Your Air supports the internal display as well as only one ThunderBolt external display. As commented below, the Mid 2012 Air can run two ThunderBolt displays. Physically you can daisy chain as many Thunderbolt Displays as you have money, but the first limitation you will run into is the graphics card.
The official Apple knowledge base article lists the capabilities of all ThunderBolt macs in one place:
First generation MacBook Airs could only drive a single display. And the current generation, Thunderbolt-equipped, MacBook Airs do not support more than one external display via the Thunderbolt port. The graphics chip used in these Macs isn't up to the task of driving extremely high resolution display combinations.
Unfortunately, this ability is lost on the 2011 MacBook Air, due to the weaker Intel HD 3000 graphics chip and a stripped down Thunderbolt controller.
What about the Sonnet Thunderbolt expansion products? These appear to be just the ticket: grab a couple of PCIe video cards, throw them in this box and voila: as much real estate as you can afford. Anyone have any actual experience with this approach?