app.js

import express from 'express';

const app = express();

if (IS_DEV) {
    require('piping')();
}
//express routes, etc.
export default app;

server.js

import app from './app';
var PORT = 8080;

var httpServer = app.listen(PORT, () => {
    console.log(`Micro-App on http://localhost:${PORT} [${app.settings.env}]`);
});

process.on('SIGTERM', () => {
    httpServer.close(() => {
        process.exit(0);
    });
});

webpack-dev-server.js

import express from 'express';
import webpack from 'webpack';
import webpackDevMiddleware from 'webpack-dev-middleware';
import webpackHotMiddleware from 'webpack-hot-middleware';
import webpackConfig from './webpack.client.babel';

const app = express();
const PORT = 8081;
const webpackCompiler = webpack(webpackConfig);
const serverOptions = {
  contentBase: 'http://localhost:' + PORT,
  quiet: true,
  noInfo: true,
  hot: true,
  inline: true,
  lazy: false,
  publicPath: webpackConfig.output.publicPath,
  headers: {'Access-Control-Allow-Origin': '*'},
  stats: { colors: true }
};

app.use(webpackDevMiddleware(webpackCompiler, serverOptions));
app.use(webpackHotMiddleware(webpackCompiler));

const httpServer = app.listen(PORT, () => {
    console.log(`HMR server on http://localhost:${PORT} [${app.settings.env}]`);
});

process.on('SIGTERM', () => {
    httpServer.close(() => {
        process.exit(0);
    });
});

webpack.config.js

    //webpack stuff
    entry: {
        main: IS_DEV ? [
            'webpack-hot-middleware/client?path=http://localhost:8081/__webpack_hmr', //same port as your webpack-dev-server.js
            './client/js/index.js'
        ] : [
            './client/js/index.js'
        ]
    },

Can you also show how do start everything together?
I believe you just spawn both webpack-dev-server.js and server.js ?