If two replacement effects apply to the same event, who chooses which applies first?

When two or more replacement effects could apply to the same event, the game must determine the order in which they apply. According to CR 616.1, if two or more replacement or prevention effects are applicable to the same event, the affected object's controller — or its owner if it has no controller — chooses which effect applies first.

After the first replacement effect modifies the event, the game checks again to see which remaining replacement effects still apply to the newly modified event (CR 616.1d). Some effects may no longer be applicable after the event has been changed, while new ones might become relevant. This process repeats until no more replacement effects can be applied.

It's important to note that if a replacement effect is controlled by a different player (for example, an opponent's effect that would affect your permanent), the affected object's controller still makes the ordering choice — not the controller of the replacement effect's source (CR 616.1).

Concrete example: You control a creature with two different replacement effects that modify how it receives damage — say, one that halves damage and one that prevents the first 2 damage. When your creature is dealt 6 damage, you (its controller) choose which replacement effect applies first. If you apply the halving effect first, damage becomes 3, then the prevention reduces it to 1. If you apply prevention first, damage becomes 4, then halving reduces it to 2. The ordering matters, and you get to decide.

If both replacement effects are controlled by different players and apply to the same event on an object neither controls, CR 616.1 specifies the affected object's controller decides. In cases of true ambiguity with no controller, the owner decides.