Hi silasmoeckel,
thanks for chiming in. Please be so kind and share some of your knowledge 
I thought the USB shield for the Arduino is just some sort of "ftdi usb - ttl serial" bridge? (As far as I understand what that is from my short search on the web.) Or am I missing an important point?
Do I need a USB shield on both ends - or can the normal USB connector (that is used for uploading the program code) be used at least on the slave side? (Sending data like if it is connected to the PC and using the serial monitor.) That would make the setup smaller and cheaper.
I will have a look at the serial library. Do you know of any resources (on the web) where I can get a bit more help with this speacial case of USB point to point connection with Arduino boards?