How to use the additional SPIs on Arduino Due
I think the information you are looking for can be found at https://www.arduino.cc/en/Reference/DueExtendedSPI

There are two other alternatives that I can see: if your device supports slave select (which I think they are supposed to), then you can use a different pin to select either device 1 or device 2 (or 3...).

The other options is that if you can connect the clock to only one slave at a time (again, with another pin), you should get the same result - you should be able to use a MOSFET between the Arduino's clock-out and the slave's clock-in, and hook the MOSFET's gate to a spare pin; repeat the procedure, using a separate MOSFET, connected to a different spare pin, to the second slave; then bring one or the other "spare pin" high, to select which way the clock goes. If a slave does not receive a clock, it will not listen for incoming signal. This last one I dreamed up; I don't know if it will work.