power supply question

This question concerns hooking up power supplies in series for higher
voltage. I have done that with a pair of 28V supplies and now have 56V.
This was with 2 identical power supplies. My question is this: can you
put another supply in series of a different voltage rating? Say, for
instance, I wanted to get closer to the 75V max voltage of my drives.
Could I use a 12V to 18V power supply in series with the 2 28V supplies?
Or do you think that I should stay away from the max voltage running
these Warner drives with NEMA 42 motors?
Carey