curl

$ curl http://localhost:8080/app1 
# logs
# main middleware
# app1 middleware

$ curl http://localhost:8080/app2
# logs
# main middleware
# app2 middleware

index.js

var express = require('express');
var app1 = express();
var app2 = express();
var app = express();

app.use(function(req, res, next) {
  console.log('main middleware');
  next();
});

app1.use(function(req, res, next) {
  console.log('app1 middleware');
  next();
});

app1.get('/', (req, res) => {
  res.sendStatus(200);
});

app2.use(function(req, res, next) {
  console.log('app2 middleware');
  next();
});

app2.get('/', (req, res) => {
  res.sendStatus(200);
});

app.use('/app1', app1);
app.use('/app2', app2);

app.listen(8080);