When the proper amount of taxes has been paid on the recording of a credit line mortgage on real property improved or to be improved by a one- to six-family owner-occupied residence or dwelling or on other real property where the credit line mortgage secures a maximum principal debt of less than $3 million, as provided in sections 647.1 and 647.2 of this Part, no further taxes are payable on the making of advances or readvances pursuant to such recorded credit line mortgage, or on the recording or filing of an instrument evidencing such advances or readvances, regardless of the cumulative total of advances and readvances thereunder, provided the advances and readvances are made to the original obligor (borrower) or to one or more of the original obligors named in the recorded primary credit line mortgage. For purposes of determining whether a credit line mortgage secures a maximum principal indebtedness of less than $3 million, two or more mortgages shall be treated as a single mortgage (and the amounts of the maximum principal debts secured by the mortgages shall be aggregated) where the mortgages form part of the same or related transactions and have the same or related mortgagors, determined in the manner prescribed by section 253-a(2) of the Tax Law and section 642.4(c) of this Title.