Apple support document HT4885:
Apple Thunderbolt Display (27-inch): Connection options for Thunderbolt Macs
MacBook Air (Mid 2011) - One Thunderbolt display
MacBook Pro (Early 2011) - Two Thunderbolt displays
iMac (Mid 2011 and Late 2011) - Two Thunderbolt displays
Mac mini (Mid 2011) - 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).
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What's odd is that Apple won't allow Mini DisplayPort daisychaining off Thunderbolt monitors, despite the fact it works with 3rd party Thunderbolt peripherals.