There will be 2 speed control loops and 2 field control loops with a reference voltage for each to match the outputs with no load.
When combining 2 voltage sources with and without a load, there be stability problems to resolve with no load and mismatched source impedance ; winding , cable resistance and field regulation impedance.
Current sensing in each source is essential for stability on current sharing which will depend on phase error for lag/lead and field voltage.
Since both outputs are ganged the voltage and current mismatch will affect stability. A flywheel may be needed for each to improve phase stability in each unless you can calculate precisely the cross generator load effects from mismatched current sharing to prevent oscillation.
This may be complex at first try but is a solvable problem if you can determine make each loop critically damped under worst case no load and full load conditions.
The loop compensation for speed must come from a common time base, with better performance using a shaft encoder rather than a zero crossing detector on voltage as inductive load transients may introduce stability errors. Several PID loops will be necessary for perfect synchronization for step load.
position is phase, velocity is the rate of phase change and acceleration is 2nd derivative of phase which is proportion to DC motor current and excess torque ( noload-load).
Begin by characterizing the phase gain block & frequency response from inertia in each component (from step response tests) and analyze like a PLL control system.
Since loading affects the closed loop response and gain, the current sense may also be used to control feedback gain and maintain a minimum of 10 dB gain margin.