Is it possible to cinnect the Arduino with a fischertechnik TX Controller? Maybe with I²C, but i want to use I²C for something else on the arduino. So are there multiple I²C Ports on the Arduino (I haven't got one yet)?
As described in the Wire library that brings supports I²C support to Arduino, except for the DUE that has 2 pairs of pins for 2 I²C buses, other Arduino boards support one I²C connection.
However, this is when you want to use hardware facilities of ATMEL chips that these limitations apply; there is a software I²C library exists that allows using other pins for I²C. One major drawback, though, is that it is very slow; not sure if this is a problem in your situation.
One important thing though: the principle of I²C is to be able to plug several slave devices (more than 100) to one master device. In general, Arduino is the master device, but the Wire library can be used as master or slave. Wouldn't that fit your requirements?
In this situation, you could just use the same I²C bus for everything, including connection to Fischertechnik TX controller.