The feature of adding multiple model nodes into a single <model-viewer>
was only conceptualized, and has not been implemented.
As stated by the Github post that you linked to:
This suggests a declarative API for incorporating multiple models into a single element (emphasis mine)...
The conversation in this Github issue goes on to describe multiple implementation suggestions if they were to incorporate the feature.
If you're looking to create a scene with multiple models, you're going to need to use a library such as three.js until they've ironed out the details of this proposed feature.
If you use ReactJS, you can use gltfjsx, which significantly simplifies the process of importing a model with multiple mesh nodes. It builds a component for you using react-three/drei, and separates out each mesh node in the component for you.
Here's an example of a component generated with gltfjsx:
import React, { useRef } from 'react'
import { useGLTF } from '@react-three/drei'
export default function Model(props) {
const group = useRef()
const { nodes, materials } = useGLTF('/example-3d.glb')
return (
<group ref={group} {...props} dispose={null}>
<group scale={56.66}>
<mesh geometry={nodes.Curve001_1.geometry} material={materials.Material} />
<mesh geometry={nodes.Curve001_2.geometry} material={materials['Material.002']} />
</group>
</group>
)
}
useGLTF.preload('/example-3d.glb')
This component can now be used anywhere to include your 3d model.