Can two replacement effects that both want to replace the same event both apply?

Replacement effects work by replacing an event before it happens, and once an event has been replaced, the new event (the result of the replacement) is what actually occurs. Because the original event never happens, only one replacement effect can apply to it directly. This is governed by CR 616.1.

However, when two or more replacement effects are all applicable to the same event at the same time, they don't simply cancel each other out. Instead, the player who is affected (usually the player whose life total, permanents, or cards are involved) chooses which replacement effect applies first (CR 616.1). That choice is made by the affected player, or if a permanent is affected, by its controller.

After the first replacement effect is applied and produces a new (modified) event, the game checks whether any remaining replacement effects are still applicable to that new event. If another replacement effect can still apply to the modified event, it may apply now — this is the process described in CR 616.1d. So in practice, multiple replacement effects can each 'take a turn,' but they do so sequentially, each applying to the result of the previous replacement.

Concrete example: A player would draw a card while both Teferi's Puzzle Box and Notion Thief replacement effects apply to that draw. The affected player chooses which replacement effect modifies the draw event first. After that replacement produces a new event, the game checks if the other effect still applies to the new event and may apply it as well — each replacement fires in sequence, not simultaneously.

Key rule references: CR 616.1 governs how multiple replacement effects interact with the same event, and CR 616.1d explains checking for further applicable replacements after each application.