追記

内部的には_.resultメソッド使ってるから、文字列を返すメソッドでもおｋ

http://backbonejs.org/backbone.js

var HogeView = Backbone.View.extend({
  className: 'aaa bbb ccc',
  tagName: 'p'
});

$(function() {  
  var hogeView = new HogeView();
  var html = hogeView.render().el;
  $('#multiple-class-demo').append(html);
});

<!DOCTYPE html>
<html>
<head>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/2.0.2/jquery.min.js"></script>
<script src="http://documentcloud.github.io/underscore/underscore-min.js"></script>
<script src="http://documentcloud.github.io/backbone/backbone-min.js"></script>
<meta charset=utf-8 />
<title>demo</title>
</head>
<body>
  <div id="multiple-class-demo"></div>
</body>
</html>

.aaa {
  outline: 5px solid #00f;
  width: 100px;
}
.bbb {
  border: 5px solid #0f0;
  height: 100px;
}
.ccc {
  background: #f00; 
}