Minecraft wiki entry for Combat, associated text: If a spider jockey spots the player from a high place, the skeleton may hurt the spider trying to shoot at the player, causing the spider to attack the skeleton, provoking a fight between them. The player can wait until one of the mobs dies so they will have to deal with a single mob or use the confusion to climb up and fight the already damaged spider jockey to get both loots. Be careful, as they could still be pushed off.