ph_test.js

var phantom = require('node-phantom-simple');
var user_agent = 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/36.0.1944.0 Safari/537.36';
var page = 'http://google.com'

phantom.create(function (phantom_err, ph) {
    ph.createPage(function (page_err, page) {
        page.open(page, function (open_err, status) {
            page.evaluate(function(){
                return navigator.userAgent;
            }, function(err, result){
                console.log(result);
                ph.exit();
            });
        });
    });
});

phantom.create(function (phantom_err, ph) {
    ph.createPage(function (page_err, page) {
        page.set('settings', {userAgent: user_agent}, function(){
            page.open(page, function (open_err, status) {
                page.evaluate(function(){
                    return navigator.userAgent;
                }, function(err, result){
                    console.log(result);
                    ph.exit();
                });
            });
        });
    });
});