Combine Multiple Ajax Call Responses Using Deferred Objects in Backbone?

Combine Multiple Ajax Call Responses Using Deferred Objects in Backbone?

How can I load in the responses for each request? What I have below doesn't work.  I think only .then will display success and error callbacks but I'm not sure how to pass in the actual responses.  How can I do this?

$.when(CountryCollection.deferred, StatesCollection.deferred)
.then(function (resp1, resp2) {
               console.log(resp1);
               console.log(resp2);
               //this callback will be fired once all ajax calls have finished.
});