For the speed limitation with one helipilot engaged, I've always assumed (although I have no positive documentation for it) that it was based on the possibility of having the one good one fail. You can fly the 412 at any speed with both disabled, but if you lose them suddenly at high speed, you're going to be very, very busy for awhile, getting it under control. The 412 is flown, theoretically, totally hands off in ATT mode, and if you lose all helipilots without warning at high cruise speed, hands off the cyclic, it's going to be very ugly.