It wouldn't necessarily be 2 conversion courses if they used a current BA Bus/737 pilot onto the 74 and filled his place with a current Bus/737 rated pilot in the hold pool (or talent pool) - it would be 1 conversion course.
That said, if they recruit rated DEP's onto the 744 it would be less training but then again I suppose you could always say that and only ever fill vacancies from rated bods if possible.