Twitter bootstrap 2个引导3.x面板之间没有槽

我需要有2个面板在引导3.x水平。我添加了如下正常代码：

<div class="panel panel-default col-md-6">
    <div class="panel-heading">Heading</div>
    <div class="panel-body">
        Something ...
    </div>
</div>
<div class="panel panel-default col-md-6">
    <div class="panel-heading">Heading</div>
    <div class="panel-body">
        Something ...
    </div>
</div>

但这两个面板之间没有排水沟，它们的外观如下所示：

---

如何解决此问题？

如何将面板设置为引导列上的子面板？

<div class="col-md-6">
 <div class="panel panel-default">
    <div class="panel-heading">Heading</div>
    <div class="panel-body">
        Something ...
    </div>
  </div>
</div>
<div class="col-md-6">
  <div class="panel panel-default">
    <div class="panel-heading">Heading</div>
    <div class="panel-body">
        Something ...
    </div>
  </div>
</div>