If damage dealt is being divided or assigned among multiple permanents or players, that damage is divided or assigned before any effects modify how much damage would be dealt. For example, say you control Collective Inferno and you attack an opponent with a 5/5 creature of the chosen type with trample and haste. If it's blocked by a 2/2 creature, you can assign 2 damage to the blocker and 3 damage to the defending player. Those amounts are then doubled to 4 and 6, respectively.