Is there a way to simultaneously use I2C and 1-Wire interfaces ? (both need open drain and there according to the user doc only 2 (IO11 and IO22 required for I2C))
One wire runs on every single pin on FEZ so yes you can use both simultaneously 
You are perfectly right. I had checked IO25 before posting and I got an exception. But I double checked after seeing your answer and figured out that I forgot the necessary pull up resistor (which is built in in my EMX dev kit on the pin used for I2C).
Thanks for your answer.