Good Grief,
I can't for the life of me find the reference to a document stating that you're not allowed to fly any other helicopter type when enrolled into type rating training course for an additional helicopter type.
JAA? EASA? Company policy?
Any takers?
You may be thinking of JAR-OPS 3.945 (8) plus associated AMC and IEM that says:
Once an operator's coversion course has been commenced, a flight crew member does not undertake flying duties on another type until the course is completed or terminated unless otherwise approved by the authority.
but that's for AOC operations not private flying, and frankly is widely ignored in the single pilot helicopter world outside the major offshore operators.