How can I route the same path for mobile & PC in Nodes.js Express

I want to do a mobile detection in express, make mobile and PC module totally independent, but keep the URL same, which means I cannot redirect a path to /mobile/... just because of the request is from mobile device.

Here is what do in app.js:

app.use(function(req, res){
    var ua = req.header('user-agent');
    if(/mobile/i.test(ua)) {
        console.log('user agent is mobile');
        mobileRouter.set(app);
    } else{
        console.log('user agent is pc');
        pcRouter.set(app);
    }
})

And in pcRouter.js:

var index = require('./pc/index');
var user = require('./pc/user');
var auth = require('./pc/auth');

module.exports.set = function(app) {
    console.log('define pc router');
    app.use('/', index);
    app.use('/user',  user);
    app.use('/auth', auth);
}

In pc/index.js:

var express = require('express');
var router = express.Router();

/* GET home page. */
router.get('/', function(req, res) {
    console.log('pc index');
    res.render('index', { title: 'PC' });
});

module.exports = router;

When I request the home page, terminal only shows these two logs: 'user agent is pc ' and 'define pc router'

pc/index.js is not being executed, I guess it's because router must be defined at the app.js been started, but how can I realize what I said before:

Same URL route by user agent detection, and keep the mobile sub app & pc sub app totally independent