If two triggers go on the stack at the same time, who chooses the order?

When multiple triggered abilities trigger at the same time and would go on the stack together, the active player (the player whose turn it is) puts all of their triggers on the stack in whatever order they choose. Then the non-active player does the same for their triggers.

Because the stack is last-in, first-out, the last trigger placed on the stack will resolve first. So the ordering choice effectively determines resolution order — the player placing their triggers chooses the order of resolution for their own triggers.

If both players control triggers going on simultaneously, the non-active player's triggers end up on top of the stack (since they're placed last), meaning they resolve first by default — unless the non-active player arranges theirs otherwise.