Mirror macbook air screen to external display and then extend that display with another external display? I have MBA mid 2013 and two 24 inch Dell external monitors. How to make mirror of laptop screen to one Dell display and then extend (or maybe I don't know which term should I use) with another Dell screen .

Like this one :
You will either need an Apple TV and a thunderbolt to HDMI/DVI/VGA cable OR some thunderbolt dock capable of multiple outputs.
And then check out the final option at the bottom here PH19039