To me this sound as the employer wants to save some money he should spend on training and rating a second (or third) pilot on his twins.
And at the same time he still wants the coverage of having a "safety" on board.
That won't work in any authority if the going gets rough.
Do yourself a favour and tell the employer:
If you want me as safety pilot, please pay my type rating for this model and I'll be happy to server you.
Otherwise you can only ask me being "flight attendant" or "passenger occupying the spare front row seat". Oh, and just in case: If it's the cost of this rating, imagine the possible earnings you can make in selling heli services with additional fully rated "safety pilot".
Well, you get the idea, I hope ;-)
Thracian