When Playing on multiplayer (to be specific I am playing on a LAN world with both instances open on the same PC) and two (different) spawners are activated by two players, it appears that either when one spawner is unloaded (the two spawners I am working with are spider and skeleton spawners, with the skeleton spawner being unloaded) while it is active (eg. by going into the nether when within 15 blocks of one of the spawners) the other spawner (when active and loaded by a second player) will spawn the mobs from the spawner that was unloaded. I don't know if this has to happen at a specific time within the spawner cycle, but I have had multiple sets of skeletons end up in my spider spawner (which definitely has no spawning spots for naturally spawned mobs, not have any other mob types spawned) (sorry for the dark screenshot)