Can you stack triggered abilities in any order when multiple triggers happen simultaneously?

When two or more triggered abilities trigger at the same time, they don't all just appear on the stack randomly. Instead, each player puts their own triggered abilities on the stack in whatever order they choose, per CR 603.3b. Because the stack is last-in, first-out, the ability you place on the stack last will resolve first.

If triggers are controlled by different players, the active player (the player whose turn it is) puts all of their triggered abilities on the stack in the order they choose first, then each other player in turn order does the same (CR 603.3b). So the non-active player's triggers end up on top and resolve first.

You cannot, however, order an opponent's triggered abilities — you only control the ordering of abilities you control. Control of a triggered ability is generally determined by who controls the permanent or effect that generated it (CR 603.3a).

Concrete example: You control both Purphoros, God of the Forge and Impact Tremors. When you play a creature, both trigger simultaneously. You can choose to put Purphoros's trigger on the stack first (resolving second) and Impact Tremors's trigger on top (resolving first), or the reverse — whatever order benefits you most.

This ordering decision can be very relevant for combat math or for ensuring one effect happens before another, so it is worth thinking through carefully whenever multiple simultaneous triggers occur.