Since LoopBack applications are regular Express applications, you can mount them on a path of the master app.
var app1 = require('path/to/app1');
var app2 = require('path/to/app2');
var root = loopback(); // or express();
root.use('/app1', app1);
root.use('/app2', app2);
root.listen(3000);
The obvious drawback is high runtime coupling between app1 and app2 - whenever you are upgrading either of them, you have to restart the whole server (i.e. both of them). Also a fatal failure in one app brings down the whole server.
The solution presented by @fiskeben is more robust, since each app is isolated.
On the other hand, my solution is probably easier to manage (you have only one Node process instead of nginx + per-app Node processes) and also allows you to configure middleware shared by both apps.
var root = loopback();
root.use(express.logger());
// etc.
root.use('/app1', app1);
root.use('/app2', app2);
root.listen(3000);