Well, with the 2012 MacBook Air you *can* do it via an Apple solution--but it won't be inexpensive. The graphics chip in the current Air supports three displays--one internal and two external. But you'd need a way to connect two monitors to the Thunderbolt port. You could do so with the Apple Thunderbolt display since it has a Thunderbolt port that passes through.
Unfortunately there are limited options for Thunderbolt aside from Apple's own.
However, here's an article on the Apple options:
http://arstechnica.com/apple/2012/06/new-macbooks-can-manage-many-many-monitors/
Note that MacBook Pros now come with multiple Thunderbolt ports, so those could do the dual monitor while in clamshell mode without having to daisy-chain via the Thunderbolt port.