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?