Many thanks for your answer and the link.
But in the FAQ it says
1 You can connect a second Apple Thunderbolt Display (27-inch) to a MacBook Pro (13-inch, Early 2011) and (13-inch, Late 2011), but the built-in display on the MacBook Pro will go dark. This is expected behavior.
That would be fine for me, since I always close the MBP when it is attached to the display.
But the question is, would my Cinema Display works when it is connected to the Thunderbold Display. Or has the second display to be a Thunderbold Display?
I can confirm that the Elgato Thunderbolt 2 Dock will work, and have read that the LaCie external hard drives with dual Thunderbolt ports also work. Apparently the Thunderbolt monitor only outputs Thunderbolt, not Displayport / video signals -- but some devices with dual Thunderbolt ports will output the video as well as pure Thunderbolt. The only way to make it work is to put a Thunderbolt device with dual Thunderbolt ports in between the Thunderbolt monitor and the Cinema Display. So the chain is Mac (Air or MBP) -> Thunderbolt cable built into Thunderbolt monitor -> Thunderbolt cable from port on back -> Elgato Thunderbolt Dock -> Apple Cinema Display's Mini Displayport cable plugged into the Elgato.