I have the same you have, of a similar age. Yes sometimes one spins slower than the other (noticeable only in light winds). Cannot find reasons for that.
If you were to have a (electric) brake switch installed that would
work by short circuiting the output wires (for D400 this brake can only be applied for very short periods, unlike for systems like AirMarine).
If you can feel that one of your units is sort of 'braked', than I guess you have a short or blown diode(s) inside the case, or external short in the
wiring maybe?
Also feel the body of each unit (when
charging, or should be charging), the warm one might have a fault. BTW a cold unit might be faulty as well due to open circuit. If not, then fault maybe then external to the unit. Does your
wiring run thought SS tubing of a stern-arch?
Insulation of wiring chafed through inside that tubing?
Best to put an amp meter on each unit and when both units have clear air, should show similar Amp output...... if not then the one with low output has something wrong with it.
Check wiring first, also remove/bypass controllers (I assume you have a controller on each?). One one them might have an internal fault and provide a short circuit, and hence the feeling that the unit is 'braked'.
If the output of both units is similar, with the one that turns slower a bit lower, then the issue might be purely mechanical, ie one of the bearings is dry-ish and provide that 'braked' feeling.