You need to redirect incoming requests to the ghost express instance. I have done so in my personal site by adding a /blog route to my primary express instance and forwarding any request to it to the ghost expresss instance. Check it out here: https://github.com/evanshortiss/evanshortiss.com/blob/master/server.js
The basic gist is that you do the following:
app.use('/blog', function(req, res, next) {
// Forward this request on...
return next();
}, ghostServer.rootApp); //...but we forward it to a different express instance
If you're running both as separate processes then you could use Apache or nginx to just redirect the requests. If you absolutely must use an express application to forward requests then try the node-http-proxy module.
If you need to proxy from express you could do this using the http-proxy module by Nodejitsu:
var proxy = require('http-proxy').createProxyServer({});
app.use('/blog', function (req, res) {
// You may need to edit req.url (or similar) to strip the /blog part of the url or ghost might not recognise it
proxy.web(req, res, {
target: 'http://127.0.0.1:'+GHOST_PORT
});
});