If damage dealt by a Wizard you control is being divided or assigned among multiple permanents and/or players, that damage is divided or assigned before doubling. For example, if you attack with a 4/6 Wizard with trample and it's blocked by a 1/1 creature, you can assign 1 damage to the blocker and 3 damage to the defending player. Those amounts are then doubled to 2 and 6, respectively.