My players have been scheming with rapid entry tactics for the megadungeon I've recently started running. Unfortunately, the monsters are aware of the adventurers, and this has lead to a weird question of priority. Given the following situation (which I'm pretty sure is going to happen next session in some form or another) what happens?
Alice the adventurer readies an action; if her teammate Carlos opens the door, she's going to throw the alchemists fire through the door, which should do enough damage to kill any goblins hiding there.
Meanwhile, on the other side of the door, Bob the goblin is lying in wait. He readies an action; if the door opens, he's going to hit the switch to open the trapdoor in the next room, which should drop any pesky adventurers into a pit with spikes at the bottom.
Carlos opens the door.
I know what happens when one readied action triggers another, but what happens when multiple readied actions get triggered at the same time? My guess would be an opposed reflex save or initiative check of some kind to see who goes first, but that's entirely just intuition speaking. The rulebook doesn't seem to speak to the issue.