They shall classify all real estate in the city in such manner, and upon such testimony as may be adduced before them, so as to distinguish between the buildings on land and the land exclusive of the buildings, and to certify to the councils of said city the aggregate valuation of city property subject to taxation. It shall be the duty of said councils, in determining the rate for each year hereafter, to assess a tax upon the buildings equal to five-tenths of the highest rate of tax required to be assessed for each such year respectively, so that upon the said classes of real estate of said city there shall, in any year, be two rates of taxation.
53 P.S. § 25894