"Bell 412 with one auto pilot disengaged max speed is 115 KIAS however, with both failed there is no specific limitation other than VNE."
I would say the restriction is for servo actuator hard-over proctection.
With 2 auto pilots operating, should one servo actuator go hard-over the other auto pilot will sense the anomaly and cause its corresponding servo actuator to motor in the opposite direction therefore mitigating the attitude excursion.
With 1 auto pilot operating a servo actuator hard-over my produce an attitude excursion that could be difficult to recover from if flying IFR.
It should say something about it in the IFR supplement for the aircraft.