Gen2/datapacks/bat-membranes/data/minecraft/loot_table/entities/bat.json

{
  "type": "minecraft:entity",
  "pools": [
    {
      "rolls": 1,
      "entries": [
        {
          "type": "minecraft:loot_table",
          "value": "vanilla:entities/bat"
        }
      ]
    },
    {
      "rolls": 1,
      "entries": [
        {
          "type": "minecraft:loot_table",
          "value": "bat_membranes:entities/bat"
        }
      ]
    },
    {
      "rolls": 1,
      "entries": [
        {
          "type": "minecraft:loot_table",
          "value": "more_mob_heads:entities/bat"
        }
      ]
    }
  ]
}