If where more than one arbiter shall have been appointed, and neither of them shall refuse or neglect to act as aforesaid, such arbiters shall fail to make their award within twenty-one days after the day on which the last of such arbiters shall have been appointed, or within such extended time as shall have been appointed for that purpose by both such arbiters under this Act, the matters referred to them shall be determined by the umpire to be appointed as aforesaid.