John Eacott's note is the key, the RFM say to start the second engine with the first at fly. Simple really.
Not to do with soft engagement and neither is the B412 from memory. The B412 is to avoid and "avoid" area.
Re taking off with one engine at idle in the A109 - it has actually led to more than one accident and even a fatality on an EMS ship in the US. This happens not only because of the engine start procedure (although a common contributing casual factor) and not because it is a PW206/207.
Have not heard of EC135 guys making the error, but I have no experience with that type.
In the A109E I would argue it is a unique human factors issue that permits take off with one engine at idle and both roof mounted throttles at fully open. In other-words, the visual indication from flying other types is the roof mounted throttles - and these can be sit ironed full open in the fly position, however the engine switches are down on the console and can be at idle even when the throttle is mismatched at fully open with no warning light.
No doubt in my mind that this is a human factors issue straight from the human factor manual of what not to do in design
And I think JE's comments on the number of guys caught out is testament to that. Shows also how poor the display system is that a Ts and Ps check is also failing to pick up one engine at idle before take off.
But this is a problem somewhat of the past if you upgrade your machine. As a result of the fatal in the US and some lobbying by other operators (we were one) the system is upgrade able to include an engine out aural warning and engine out light which activates anytime you increase collective with one engine at idle.
Engineering out the problem is always the superior answer to human factor issues, and short of getting rid of the whole throttle design in the A109E, they have done a good job with this one. It is a very effective engineering solution and all A109E operators should do it or face the almost inevitable instance of making the same mistakes and risking the same old outcomes from the same old known issue.
Not sure if this affects the grand and grand new, I have no experience in those types.