Webpack4 ExtractTextPlugin にて複数のcssファイルを出力したい

Wordpressのtheme開発にWebpack4を導入、ExtractTextPluginを使い wp-content/themes/project-name/assets/css フォルダへ main.css を出力しています。

wordpressの管理画面用css として editor-style.css を同出力先へ、 main.css とは別のファイルとして、新たに書き出したいと考えているのですが、うまくできません。

一台の webpack で複数の css ファイルを出力する方法をご教授頂ければ幸いです。

現在使用しているwebpack-config.jsは以下です。

webpack-config.js


const ExtractTextPlugin = require('extract-text-webpack-plugin')

const MODE = 'development'
const enabledSourceMap = (MODE === 'development')

const path = require('path')

const WP_THEME_NAME = 'project-name'
const WP_THEME_PATH = '../wordpress/wp-content/themes/' + WP_THEME_NAME
const PUBLIC_DIR_PATH = path.resolve(__dirname, '../wordpress')
const PUBLIC_DIR_DIST_PATH = path.resolve(__dirname, WP_THEME_PATH)
const PUBLIC_ASSETS_DIR_PATH = './assets'

module.exports = {
  mode: MODE,
  entry: `./src/js/index.js`,
  output: {
    path: PUBLIC_DIR_DIST_PATH,
    filename: PUBLIC_ASSETS_DIR_PATH + '/js/app.js'
  },
  devtool: 'source-map',
  devServer: {
    contentBase: PUBLIC_DIR_PATH,
    open: true,
    host: '0.0.0.0',
    public: '0.0.0.0',
    disableHostCheck : true,
  },
  module: {
    rules: [
      {
        test: /.scss/,
        use : ExtractTextPlugin.extract({
          use: [
            {
              loader: 'css-loader',
              options: {
                url: true,
                sourceMap: enabledSourceMap,
                minimize: true,
                importLoaders: 2
              },
            },
            {
              loader: 'postcss-loader',
              options: {
                sourceMap: true,
                plugins: [
                  require('autoprefixer')({grid: true})
                ]
              },
            },
            {
              loader: 'sass-loader',
              options: {
                sourceMap: enabledSourceMap,
              }
            }
          ],
        }),
      },
      {
        test: /.(gif|png|jpg|eot|wof|woff|woff2|ttf|svg)$/,
        loader: 'url-loader'
      },
      {
        test: /.js$/,
        use: [
          {
            loader: 'babel-loader',
            options: {
              presets: [
                ['env', {'modules': false}]
              ]
            }
          }
        ],
        exclude: /node_modules/,
      }
    ]
  },
  plugins: [
    new ExtractTextPlugin(PUBLIC_ASSETS_DIR_PATH + '/css/main.css' ),
  ],
}

###使用中パッケージ

    "autoprefixer": "^8.6.4",
    "babel-core": "^6.26.3",
    "babel-loader": "^7.1.4",
    "babel-preset-env": "^1.7.0",
    "css-loader": "^0.28.11",
    "extract-text-webpack-plugin": "^4.0.0-beta.0",
    "node-sass": "^4.9.0",
    "postcss-loader": "^2.1.5",
    "sass-loader": "^7.0.3",
    "style-loader": "^0.21.0",
    "url-loader": "^1.0.1",
    "webpack": "^4.12.2",
    "webpack-cli": "^3.0.8",
    "webpack-dev-server": "^3.1.4"