Based on @Michaaatje and @papiro, a very easy way:
Say you have some typical pages such as...
var app = express()
app.use(sess)
app.use(passport.initialize())
app.use(passport.session())
app.use('/static', express.static('static'))
app.get('/', ensureLoggedIn("/loginpage"), function(req, res, next) {
    ...
})
app.get('/sales', ensureLoggedIn("/loginpage"), function(req, res, next) {
    ...
})
app.get('/about', ensureLoggedIn("/loginpage"), function(req, res, next) {
    ...
})
app.post('/order', ensureLoggedIn("/loginpage"), urlencodedParser, (req, res) => {
    ...
})
.. and so on.
Say the main domain is "abc.test.com"
But you have an "alternate" domain (perhaps for customers) which is "customers.test.com".
Simply add this ...
var app = express()
app.use(sess)
app.use(passport.initialize())
app.use(passport.session())
app.use('/static', express.static('static'))
app.use((req, res, next) => {
    req.isCustomer = false
    if (req.headers.host == "customers.test.com") {
        req.isCustomer = true
    }
    next();
})
and then it is this easy ...
app.get('/', ensureLoggedIn("/loginpage"), function(req, res, next) {
    if (req.isCustomer) {
        .. special page or whatever ..
        return
    }
    ...
})
app.get('/sales', ensureLoggedIn("/loginpage"), function(req, res, next) {
    if (req.isCustomer) {
        res.redirect('/') .. for example
        return
    }
    ...
})
app.get('/about', ensureLoggedIn("/loginpage"), function(req, res, next) {
    if (req.isCustomer) { ... }
    ...
})
app.post('/order', ensureLoggedIn("/loginpage"), urlencodedParser, (req, res) => {
    if (req.isCustomer) { ... }
    ...
})
Thanks to  @Michaaatje and @papiro .