Complex sentence: Philibert de L'Orme's pre-existing northern pavilion was used for an auditorium holding up to 4,000 spectators, while a northern extension, designed by Louis Le Vau and François d'Orbay, was built to accomodate the enormous stage.
Split into simpler sentences: Philibert de L'Orme's pre-existing northern pavilion was used for an auditorium holding up to 4,000 spectators. It was designed by Louis Le Vau and François d'Orbay, who also added the north-wing extension which accommodated the enormous stage.