Thanks for your reply Paul
I actually have a mySerial connected to the pins yes. My RFID is connected there. As the only thing the RFID needs is the RX pin, I've just selected a unused pin (A2) as the TX pin as I can't just leave it blank.
I need them to talk to the pc, and therefore I can't use pin 0 and 1. Actually I would be glad is I could just listen to the RFID as default, and then only listen to the SIM900 while sending a message, and then go back and listen to the RFID until a new SMS has to be sent. But as I said I am unable to figure out how to change between the software serials as the SIM900 library doesn't support the command "gsm.listen();" as I would have used then I had to listen at the SIM900 instead of the RFID.
best regards