Currently, when you use a datapack/mod to replace a loot table, and another datapack/mod replaces the same loot table, the last to load will have priority. However, as seen in tag files within datapacks, its perfectly possible to add a new Entry for a loot table that will simply use the previous version of the loot table, before yours replaced it. For exsample, if you wish to add diamonds to whatever loot table was there before your datapack was installed, you can now do:

{
  "type": "minecraft:generic",
  "pools": [
    {
      "rolls": 1,
      "entries": [
        {
          "type": "minecraft:item",
          "name": "minecraft:diamond"
        },
        {
          "type": "minecraft:self"
        }
      ]
    }
  ]
}
 

This is just a example for one possible implementation. However, adding such a feature would make compatibility between datapacks a lot easier to be done, and make it so we do not need to look into Minecraft's default loot tables, and copy large amounts of text just to add 1 new item to a loot table. Just so that it will all go to waste once another datapack overwrites it. Instead, both can work.