portal.js

import React, { Component } from "react";
import { render, createPortal } from "react-dom";

class List extends Component {
  render() {
    return [
      <li key="A">First item</li>,
      <li key="B">Second item</li>,
      <li key="C">Third item</li>,
      <OtherList />
    ];
  }
}

class OtherList extends Component {
  render() {
    return createPortal(
      [
        <li>Fourth item</li>,
        <li>Fifth item</li>
      ],
      document.getElementById("root2"),
    );
  }
}

render(<List />, document.getElementById("root"));