Need assistance with a problem I am experiencing on my CNC machine

I am running the Y axis of my CNC machine using two Nema 23 stepper motors (51mm) connected in series off a MKS TB6600 4,5 amp stepper driver. Power supply 12v, 8.5 amps. The problem I have is that the two stepper motors seem to run at different speeds, as a result the gantry pulls skew and the cuts are not square. I have to regularly square the gantry. Has anyone else experienced this when running two motors in series and if so suggest how this can be corrected.