How can I get socket.io to work with subdomains?

I'm running a node application that conditionally .uses other applications based on the subdomain of the HTTP request. The code for that is as below:

var server = http.createServer(app);

...

global.socket = io.listen(server);

...

subdomains.forEach(function (subdomain) {
    var vhost = (subdomain == '_' ? '' : subdomain + '.') + domain;
    app.use(express.vhost(vhost, require('./' + subdomain)));
});

Now suppose I want to use socket.io in two different subdomains. They both have access to the same socket, since it's running on the same port. Yet if I try to connect to http://sub1.domain.tld/, any broadcasts are also sent to http://sub2.domain.tld/.

Is there any way I can restrict broadcast, emit, &c. based on the subdomain of the connection?