Hi Patrick,
I have tested to check that both the motors and drivers work when there is no other motor or driver in the circuit and they both do. I don’t have an oscilloscope to test unfortunately, one day!
However when testing with both motors and drivers in the circuit, the 1st motor always works as expected, but the 2nd motor whines, then when it hits the section where it should turn, the whining seems to stop, and the shaft does tiny movements that arent coded, the 2nd motor it is heating up while this is happening so I unplug.
When I remove the 24V power from the 1st driver (all other SPI and digital connections remain in the circuit) the 2nd motor whines, then when the loop hits the section where it should turn, the whining changes tone, but it doesnt produce the tiny movements like before. Again the 2nd motor heating up here so I unplug.
Thankfully nothing has burnt out yet! 
Here are a couple photos of my setup - as i mentioned, using the same wiring as Dave. The stipboard is allowing me to use the same ground, ref voltage and MISO, MOSI and SCK pins.
