CS/SS pin configuration
I believe you can use any GPIO pin as CS/SS. Keep in mind you will need a different pin for each slave device. For example say device0 SPI CS is connected to PORTD pin 5, and device1 SPI CS is connected to PORTD pin 6:

//init ports    
DDRD  |= (1 &lt;&lt; 5) | (1 &lt;&lt; 6);    //sets PORTD bits 5 and 6 to output
PORTD |= (1 &lt;&lt; 5) | (1 &lt;&lt; 6);    //sets PORTD bits 5 and 6 to high (spi CS inactive)

//spi device0
PORTD &amp;= ~(1 &lt;&lt; 5);    //resets device0 CS (makes it active)
//SPI send/receive stuff here
PORTD |= (1 &lt;&lt; 5);     //sets device0 CS (makes it inactive)

//spi device1
PORTD &amp;= ~(1 &lt;&lt; 6);    //resets device0 CS (makes it active)
//SPI send/receive stuff here
PORTD |= (1 &lt;&lt; 6);     //sets device0 CS (makes it inactive)