Where, for the purposes of this Chapter, any sum falls to be apportioned and, at the time of the apportionment, it appears that it is material as respects the liability to tax (for whatever period) of two or more companies, then any question which arises as to the manner in which the sum is to be apportioned shall be determined, for the purposes of the tax of all those companies, by the Appeal Commissioners, who shall determine the question in like manner as if it were an appeal against an assessment, and the provisions of the Income Tax Acts relating to the rehearing of an appeal and the statement of a case for the opinion of the High Court on a point of law shall apply accordingly with any necessary modifications:
Provided that all the said companies shall be entitled to appear and be heard by the Appeal Commissioners or to make representations to them in writing.