Create root XML node with more than 1 namespace

I am working with Backbone.js with Restful services. Have to post XML. I want to add more than 1 namespace

Current JS code is like,

var nsp = "xmlns='http://ift.tt/19n5AFC'";
var nsp2 = "xmlns:ns2='http://ift.tt/19n5AFE'";

var doc = document.implementation.createDocument(nsp, "ns2:subscription", "");

But i expect XML root node is like,

<ns2:subscription xmlns='http://ift.tt/19n5AFC' 
xmlns:ns2='http://ift.tt/19n5AFE'>..</ns2:subscription>

Thanks in advance.