Motor or Generator?

Are the voltages/currents being measured under LOAD? and is there a Load Sharing lead connecting the two plants? You might want to have a load bank test done on them individually and then go from there. As a rule of thumb, when two or more plants are running in parallel, they SHOULD share the load EQUALLY, if they are not, give the manufacture's Tech support a holler. They are usually very helpful..

Kilowatt hour meters indicate import or export which will indicate what is actually going on. If one imports read above, something is wrong, usually engine tuning.

I agree with kWh meter principles you mentioned...I guess that I don't have a solid acquisition on what the circumstances surrounding the original problem... We (US Army) used to run 60Kw gen sets in parallel and monthly we were required to run a load-bank test. If everything did not go according to the parameters indicated in the operators' manual, then we had to call in the 'tie-guys' (civilian contractors) and they would tweak and push and pull and replace until every gen set was within required limits... Hey, just a thought...

Its usual to connect a reverse power relay to each generator to prevent the generator becoming a motor and turning your prime mover. The problem is that the speed regulation of the prime mover is defeated and this can cause mechanical damage due to overspeeding

the short answer is that motoring only occurs when current flows in the opposite direction as generating.
If one generator lowers in output voltage, it will reduce or reverse the generating effect. When a generator turns, it produces an electro-magnetic flux(?) (EMF) which essentially is a voltage source. Current that flows as a result goes through the resistance of the phase winding wire and so the measured voltage at the terminal under loaded current flow will be reduced becuse of that phase winding.
So it really depends on where and how you measure the reduced voltage.
Hope that helps.

Just out of curiosity, why would you connect two generators together? Is one load (only load?) too large for one generator? Such is done regularly at power plants, but they have phasing and voltage regulation equipment. Mathematical calculations to determine exactly what's going on when two generators are connected together is complex, to say the least. If the current between generators is half of the total of the two going to the load, then one can guess that each is providing half the current. Without ammeters, voltmeters, and oscilloscopes, and knowledge of how to use them, however, one cannot be sure exactly.

To share heavy loads Generators & UPS Modules are connected in paralle through very spedial Syncronizing Circuits.
They are capable to share load on all connected in parralel, sync Freq, output voltage, Phase-shift [power-factor] etc.

The Syncing system does not allow to happen this unless it is defective. Syncronysing Circuits arrange to divide load propotionately to all the devices of power [Gen or UPS Modules in abundanse] to share theload & not to become load.
If such condition occurs the defective one is ISOLATED by the system.

THIS KIND OF SITUATION CAN BE BEST SOLVED BY HAVING A VOLTMETER. the terminal voltages of the two machines should be found out and judged.as current always flows from high potential; pioint to lower potential point, the one at high voltage is the generator and one at lower potential receives the current is the motor.

Looking at a voltmeter, ammeter, and wattmeter for each generator ( A, and B ).

If the voltage from A is gradually reduced it's watts will remain constant, but amps will increase because the imaginary or out of phase current is increasing. As A's voltage is continually decreased it's amps will continually increase. If there is over-current protection A will trip. If there is speed control on A it will not change speed. The two generators become extremely out-of phase which appears as excessive current. With no overcurrent protection, something in the A circuit will smoke.