Complex sentence: After graduating from the Université de Montréal, he studied at the University of Paris where he met Pierre Trudeau, with whom he founded the dissident political magazine "Cité Libre" upon returning to Montreal.
Split into simpler sentences: After graduating from the Université de Montréal, he studied at the University of Paris where he met Pierre Trudeau. He and Trudeau whom he founded the dissident political magazine "Cité Libre" upon returning to Montreal.