Can two external displays be driven from one Thunderbolt port?
Solution 1:
While Apple's display adapters will let you connect one of these external monitors, there's also the Matrox DualHead2Go Digital ME which will connect two external DVI displays (I've never seen or used one, ymmv).
Solution 2:
The current-generation MacBook Air's integrated graphics only supports one external display.
Further, there are no products that I know of that are shipping (as of when I answered this) that can "split" a Thunderbolt signal to connect two non-Thunderbolt displays. The only way I know of to connect multiple displays to a Thunderbolt Mac via Thunderbolt is to use Apple's Thunderbolt Display.
You could use one of the various DisplayLink products to connect one of your displays via USB and use a plain mini DisplayPort adapter for the other display (this would connect to your Thunderbolt port).
Solution 3:
Zotac provides an adapter to split a regular or mini DisplayPort into two HDMI ports. It's the ZT-DP2HD (DisplayPort) or ZT-MDP2HD (mini DisplayPort). Anandtech has a brief blurb about it here. For $50, I think it's worth checking out.