Sounds like you are going for 'digital control'. :-)
The max speed must be more than enough to satisfy the process. Easiest way is to have one thermostat controlling the low speed function and a second TS bringing on the high speed winding if the motor low speed is insufficient (after a short...
If the transformers are identical in all aspects other than the 1% impedance difference there will be no open cct voltage difference on the low sides before parallelling.
The impedance difference will mean that one transformer will carry more load than it's higher impedance twin, so use CTs...
