Split Model Items

Registration

Inside your custom item class add impliments SplitModelItem

Eg:

public class TestItem extends Item implements SplitModelItem {
    public TestItem(Settings settings) {
        super(settings);
    }

    @Override
    public String getModelBaseName() {
        return "test_item";
    }
}

Replace test_item with the name of the items .json file

Different Models

Inside /resources/assets/modid/models/item you will need itemname_gui.json and itemname_hand.json leading to the different textures. Examples:

{
  "parent": "minecraft:item/generated",
  "textures": {
    "layer0": "divine_treachery:item/test_item_gui"
  }
}

{
  "parent": "minecraft:item/generated",
  "textures": {
    "layer0": "divine_treachery:item/test_item_hand"
  }
}