608.5b.

If a player has priority for multiple stacks and plays a spell or ability, or a triggered ability he or she controls triggers, the player must specify which one of those stacks the spell or ability is put on. If an object on one of those stacks caused the triggered ability to trigger, the player must put it on that stack. If a resolving spell or ability on one of those stacks causes a player to play a spell or create a copy of a spell, the new spell must be put on the same stack. If a spell or ability targets an object on one of those stacks, it must be put on the same stack as its target; it can't target objects on multiple stacks.