If two triggered abilities from different players trigger at the same time whose goes on the stack first?

When multiple triggered abilities trigger at the same time, they are placed on the stack in a specific order determined by the rules. First, the active player (whose turn it is) puts all of their triggered abilities on the stack in whatever order they choose. Then, each non-active player, in turn order, puts their triggered abilities on the stack in whatever order they choose. (CR 603.3b)

Because the stack resolves last-in, first-out, the abilities that were placed on the stack last resolve first. This means the non-active player's triggered abilities will resolve before the active player's, since they were placed on top.

If there are more than two players, each non-active player adds their triggers in turn order after the active player, so the player furthest from the active player in turn order will have their abilities resolve first.

Concrete example: It's your turn (you are the active player). Both you and your opponent control a Grim Lavamancer, and a card went to each graveyard simultaneously. Both Landfall/trigger effects trigger at the same time. You place your trigger on the stack first, then your opponent places theirs on top. Your opponent's trigger resolves first.

Note: each player chooses the order of their own simultaneous triggers (CR 603.3b), giving some control even within this framework.