I don't understand your confusion.
Say you had two devices doing SPI comm's, both connected to 11-12-13, one to 10 for SS and one to 4 for SS.
Pin 6 has nothing to do with either in your description.
You could read from it and write to it with out bothering the SPI devices.
Each SPI.transfer( ) will be accompanied by a burst of clock pulses on the SCK pin, no transfers occur otherwise.