So is a training Captain giving Instruction for the pupose of the issue of a licence or rating a you suggest?
He would need a multi instructors rating to give instruction on the candidates first multi engine aircraft.
The requirement for training on subsequent types is a matter for the CAA and the operator to decide and agree and publish in the operations manual.
The operations manual could be considered to be the law but only for operations within that airline, they could in fact write any requirement in that they like but of course it has to be approved by the CAA before the AOC is issued.
If a PPL wanted to put a 747 on his licence he would have to have a pilot with an instructors rating and a type rating on the 747.
But subsequently
any pilot could give that PPL instruction on a 747, he would need a 747 rating of course