Provided always, and be it enacted, that if it shall be made to appear to the Commissioners of Stamps and Taxes that any Two or more Banks have, by written Contract or Agreement (which Contract or Agreement shall be produced to the said Commissioners), become united within the Twelve Weeks next preceding such Twenty-seventh Day of April as aforesaid, it shall be lawful for the said Commissioners to ascertain the average Amount of the Notes of each such Bank in the Manner herein-before directed, and to certify the average Amount of the Notes of the Two or more Banks so united as the Amount which the united Bank shall thereafter be authorized to issue, subject to the Regulations of this Act.