Complex sentence: Instead he went into partnership with William Martin who was already established as the city's public works architect, with Chamberlain taking the lead in design matters and Martin seeing to the more practical side of running an architectural practice.
Split into simpler sentences: Instead he went into partnership with William Martin who was already established as the city's public works architect. Chamberlain took the lead in design matters, while Martin saw to the more practical side of running an architectural practice.