How can I get the diopter distance of WebXR cameras?

I would like to get the positions of the two (or more) cameras used for rendering 3D via WebXR. I tried this snippet of code, but I always get zero cameras in the array:

    const s = this.renderer.xr.getCamera(camera) as THREE.ArrayCamera;
    if (s && s.isArrayCamera) {
      if (!this.cameras || this.cameras.cameras.length < s.cameras.length) {
        this.cameras = s;
        Debug.log(`Camera count: ${this.cameras.cameras.length}`)
        for (const c of this.cameras.cameras) {
          Debug.log(`X: ${c.position.x}`);

My end goal here is to make something like the portal demo work in WebXR.

Thank you!