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Not sure if this answers your question, but I just tested the spare 6.7" GE 3.5 HP series motor I have. Here are the results:
Field resistance: .22 ohms
Armature resistance: .34 ohms (varies as the armature is turned, but generally settles in to around .34 when not moving)
Using a 12v power source from a portable jumper battery pack (12v, 19aH gel battery) the motor turns about 3,000 RPM unloaded when configured in series. (power in at A1, A2 and S2 jumpered, power out at S1 - this motor 'normal' rotation is reverse from most other motors).
Using this battery, the voltage across the armature posts (A1, A2) is 11.31 volts. Voltage across the field is .191 volts. Running voltage from the battery + and - is 11.47 volts. Amperage was not measured, but probably about 40 amps.
In simulating two controllers (one battery to the field, another to the armature) the motor would not self-start, and the brushes made complaining noises and got hot.
Starting the motor in series mode up to about 3,000 RPM, I quickly reconfigured the power as the motor coasted down, with one 12v battery powering the field, and another powering the armature. This time, the motor accelerated to much faster than 4,000 RPM (estimate), probably because the field battery happened to be weaker than the armature battery (quasi field weakening).
Based on these observations, I believe the two controller setup will not start the motor from a dead stop, but could be useful to reduce back EMF in order to make a fully loaded motor run faster at relatively high RPM. It would be complicated to make a second controller weaken the field enough to control the motor RPM and not induce too much current across the armature. Probably much easier to just make up a simple weakening resistor that is switched in and out as needed.
Additional complications may be synchronizing the timing pulses of the two controllers, although at 20 kHz (artificial DC to a motor) it may not make a difference.
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