Complex sentence: Between agreement and adoption of the plan came the Wall Street Crash of 1929 which main consequences are twofold: the American Banking system had to recall money from Europe and cancel the credits that made possible the Young Plan.
Split into simpler sentences: Between agreement and adoption of the plan came the Wall Street Crash of 1929, of which the main consequences were twofold. The American Banking system had to recall money from Europe and cancel the credits that made possible the Young Plan.