The TMC's can handle far more current then the steppers. What is the correct current rating for the steppers? The value on the small label on the stepper stating 1,5A or the value from the blurred specsheet on the FT-website stating 0,4A. Is the first one something about peak current and the other nominal/continuous/RMS current? I don't know. If it is 0.4 RMS then peak would be 0.4 times 1,4142 (sqrt of 2) is 0,56 amps.
Anyway : you could give each stepper it's own driver with the duet+stepper expansion board.
Now I handle Z and Y with 1.3 amps for 2 steppers. If they behave identically (electrical point of view) current per stepper would be half of that.
Some more confusion : FT5 specs on FT website : x6 NEMA 17 5.0kg/cm Stepper Motors
Torque is specified in kg.cm or Nm but not certainly not kg/cm.
If it is 5kg.cm then this would be 49N.cm
The max torque on the 'specsheet' is 260mN.m (milli Newton meter) which is 2,6 kg.cm
So draw your conclusions ...