Based on my experience, you will have a hard time getting the two units to share load unless the control systems are tied together. A slight difference in voltage output can switch the load totally to one rectifier. If the units have a droop setting (output voltage drops as the unit loads up) you may be able to tune the droops to achieve load sharing at steady state.
A load sharing circuit on the AC input sometimes works, if it can be added into the thyristor firing circuit control. A CT on each rectifier’s AC input with the two sets of CT secondaries wired in series anda loading resistor across each CT secondary winding is one way of getting a load sharing signal. At equal loading, the current circulates in the CT secondary windings and no current flows in the resistors. If one rectifier backs off load, the excess CT secondary current is forced to flow through that unit's resistor, generating an AC voltage that can be used to adjust the controls to rebalance.
Otherwise, the two units may work OK together. Since they are traction units, their design probably protects against surge voltages on the DC side caused my motor switching. My gut feeling is that the electrical noise and spikes from a rectifier will be less than those caused by a motor.
To reduce input harmonics, you might want to look at feeding one rectifier from a delta-delta transformer and one from a delta-wye. That will give an extra 30 degree phase shift and make a 12-pulse power supply.
