How to use multiple SPI slaves in MKR1000 if it doesn't have Slave Select (SS/CS) pin?
The Arduino doesn't care what pin you use for SS since you just use digitalWrite() on it anyway. You use any pin you like for one slave, so you can use any other pin you like for another slave.

The ATMega-based boards need pin 10 to be set as an output for SPI to function, but you don't then have to use that for the actual slave select. The only time it becomes a real slave-select pin is when using the Arduino itself as a slave.