Where more than one arbiter shall have been appointed such arbiters shall, before they enter upon the matters referred to them, nominate and appoint by writing under their hands an oversman to decide on any such matters on which they shall differ, or which shall be referred to him under the provisions of this or the special Act; and if such oversman shall die, or become incapable to act, they shall forthwith after such death or incapacity appoint another oversman in his place; and the decision of every such oversman on the matters on which the arbiters shall differ shall be final.