If two triggered abilities trigger at the same time controlled by different players, who puts theirs on the stack first?

When multiple triggered abilities trigger simultaneously and are controlled by different players, the order they are placed onto the stack is determined by turn order. The active player (the player whose turn it currently is) puts all of their triggered abilities on the stack first, in whatever order they choose. Then each other player, in turn order, does the same. (CR 603.3b)

Because the stack is last-in, first-out, the abilities placed on the stack last will resolve first. This means the non-active player's abilities, being placed on the stack after the active player's, will generally resolve before the active player's abilities — giving the non-active player a slight 'speed advantage' in simultaneous triggers.

Each player may order their own simultaneous triggers in any order they wish before putting them on the stack. So if you have two triggers and your opponent has one, you decide which of your two goes on top of yours, then your opponent's goes on top of everything. (CR 603.3b)

Concrete example: It's your turn (you are the active player). Both you and your opponent have a creature with an 'enters the battlefield' trigger that fires at the same time. You place your trigger on the stack first, then your opponent places their trigger on top. Your opponent's trigger resolves first, then yours.

This rule is found in CR 603.3b, which references the general rule for handling multiple triggers in CR 101.4 (the Active Player, Non-Active Player — APNAP — rule).