index.js

const textureLight = textureLoader.load('/textures/matcap-light-512.png')
const textureDark = textureLoader.load('/textures/matcap-dark-512.png')

const materials = {
    light: new THREE.MeshMatcapMaterial({ matcap: textureLight }),
    dark: new THREE.MeshMatcapMaterial({ matcap: textureDark }),
}

const updateAllMaterials = (scene) =>
{
    scene.traverse((child) =>
    {
        if(child instanceof THREE.Mesh)
        {
            const type = child.name.split('_')[0]
            if(materials[type]) child.material = materials[type];
            else console.error(`This mesh name was missing the material type`, child.name)
        }
    })
}

const models = [
  {file: 'model1.glb'},
  {file: 'model2.glb'}
]

models.forEach(model => 
{
    gltfLoader.load(
        `/models/${model.file}`,
        (gltf) =>
        {
            scene.add(gltf.scene)
            updateAllMaterials(gltf.scene)
        }
    )        
})