Thanks to the comment from Bas van Stein, here is a complete working example.

    var restify = require('restify');
    var fs = require('fs');

    // Setup some https server options
    //generated from http://www.selfsignedcertificate.com/
    var https_options = {
        key: fs.readFileSync('./HTTPS.key'), //on current folder
        certificate: fs.readFileSync('./HTTPS.cert')
    };

    // Instantiate our two servers
    var server = restify.createServer();
    var https_server = restify.createServer(https_options);

    // Put any routing, response, etc. logic here. This allows us to define these functions
    // only once, and it will be re-used on both the HTTP and HTTPs servers
    var setup_server = function(app) {
        function respond(req, res, next) {
            res.send('I see you ' + req.params.name);
        }

        // Routes
        app.get('/test/:name', respond);
    }

    // Now, setup both servers in one step
    setup_server(server);
    setup_server(https_server);

    // Start our servers to listen on the appropriate ports
    server.listen(9848, function() {
        console.log('%s listening at %s', server.name, server.url);
    });

    https_server.listen(443, function() {
        console.log('%s listening at %s', https_server.name, https_server.url);
    });