I don't know the answer but you would have two machines in parallel, both trying to act like generators. The results would depend on the control systems and protective relaying.  I assume the generator controller tries to maintain voltage and frequency.  As motor p[ower output (to electrical system) increases, the generator will speed up and reduce it's mechancial input power in attempt to maintain frequency. If motor continues to try to put power into electrical system when generator mechanical input approaches it's minimum value (?zero), then I can only imagine a few possible scenario's:
1 - If equipped with reverse power protection the generator would trip.
2 - If reverse power is not equipped , the generator will trip on overspeed.
That's just my thoughts from limited experience with small generators and no experience with regenerative loads. Any other comments?

I would suggest you avoid this situation and put in resistors for dynamic braking instead of back feeding into the generator.

perplexed:

Speak with the guys at Bonitron...they have some great information about regeneration and portables.

http://www.bonitron.com/

Carl

I've had some experience in regen braking on RTG cranes with diesel generator plant, where the resistive dump bank has not been sized sufficiently.
The net result is an increase in genset speed as the set goes into reverse power mode.
In one instance in the UAE, a 1500kVA D/G set went to 2050RPM during the 40 ton lower brake test.
The reverse power operation tends to cause high circulating current in the generator rotor damper winding. In this case the heating lasted only for around 40 seconds, but prolonged periods of damper cage heating will cause failure, usually ath the damper lam corners, where the section is at a minimum.
As others responding, I would suggest you add or increase resistive load banks to absorb the generated power and negate the reverse power problem.
Hope this helps