I talked to the embasy now and they told me that if i exstend a multiple entry TR visa then the extension is counted as a different visa. Since you only allowed to hold one visa at the time the TR visa will be invalid after extension.
Furthermore the ambassador recommended me to get a single entry visa and told me if i ask the immigration office nicly to extend it for more than 30 days they might be able to sort me out...
I love Thailand
*****