In the county of Allegheny, for the purpose of first organization under this Constitution, the judges of the court of common pleas, at the time of the adoption of this Constitution, shall be the judges of the court number one, and the judges of the district court, at the same date, shall be the judges of the common pleas number two. The president judges of the common pleas and district court shall be president judge of said courts number one and two, respectively, until their offices shall end; and thereafter the judge oldest in commission shall be president judge; but any president judge re-elected in the same court, or district, shall continue to be president judge thereof.