Minecraft wiki entry for Spawner_traps, associated text: To maintain maximum efficiency a trap must take on an increased complexity in keeping mobs separated or moving quickly when dealing with multiple spawners of the same species. Minecraft first checks within the 17 wide by 17 long by 9 high area with the spawner in the center (or 8×8×4 radius from the spawner) for mobs of the same species as the spawner. If there are fewer than 6 mobs of the same species in the checking zone, it will spawn a mob within half the distance of the checking radius. This means that with two spawners of the same species near to each other, there will be a zone that mobs will count for both spawners and severely impact the spawn rates. It would be a wise idea to work out where these checking zones overlap and ensure that mobs being funneled to the trap do not enter that zone and/or leave the overlapped/checking area as quickly as possible to maintain peak monster spawner output.