6Where any company,- either in its own name or that of a nominee, owns the whole of the ordinary capital of any other company carrying on the same trade or business or so much of that capital as under the general law a single shareholder can legally own, the provisions of Part III of this Act as to excess profits duty and the pre-war standard of profits shall apply as if that other company were a branch of the first-named company, and the profits of the two companies shall not be separately assessed.