Will the Macbook Air (Mid 2012) support dual monitors?
You can hook up two external monitors by using the thunderbolt port that support daisy chaining. You connect the the first screen with your Macbook air and connect the second screen with the first screen.

According to the following KB-article the following models support up to 2 displays through one thunderbolt port. 


MacBook Pro (Retina, 13-inch, Late 2012) and later
MacBook Pro (Retina, Mid 2012) and later
MacBook Pro (13-inch, Mid 2012)
MacBook Pro (15-inch and 17-inch, Early 2011) and later
MacBook Air (Mid 2012) and later
iMac (Late 2012)
iMac (Mid 2011)
Mac Mini (Late 2012) 
Mac Mini (Mid 2011), 2.5 GHz


It's not possible to connect 2 monitors simultaneously through one hdmi port.



Arstechnica also has an article about the support of multiple external monitors through one thunderbolt port. Apparently the older Macbook airs only support one external screen due to limitations in the thunderbolt host controller.