helpers.js

Ember.Handlebars.registerHelper('partition', function(path, options){
  path = path === '' ? 'this' : path;
  var arr = [];
  var data = Ember.Handlebars.get(this, path, options).content;
  var ret = '';
  while(data.length){
    arr.push(data.splice(0, options.hash.size || 2));
  }
  for(var i = 0, len = arr.length; i < len; i++){
    ret = ret + options.fn(arr[i]);
  };
  return ret;
});

template.hbs

<h2>Your photos</h2>
{{#partition controller.photos size=2}}
  <div class="slide">
    {{#each this}}
      {{this.title}}
    {{/each}}
  </div>
  <br/><br/><br/>
{{/partition}}