According to the document the MacBook Air (Mid 2011) only supports one Thunderbolt Display; whereas, the MacBook Pro (Early 2011), iMac (Mid 2011 and Late 2011), and Mac mini (Mid 2011) support two Thunderbolt displays.
1. Connecting a second Apple Thunderbolt Display (27-inch) to a 13-inch MacBook Pro will make the screen on the MacBook Pro turn black. This is expected behavior.
2. iMac (27-inch, Mid 2011) with two Thunderbolt ports supports a total of two Thunderbolt displays regardless of which Thunderbolt port each display is connected to.
3. Mac mini with AMD graphics can support a HDMI compatible device on its HDMI port when using two Thunderbolt displays.
Mini DisplayPort displays will not light up if connected to the Thunderbolt port on an Apple Thunderbolt Display (27-inch). For best performance while using Thunderbolt storage devices, connect the Thunderbolt display to the computer's Thunderbolt port first.
Read More [via MacRumors]