Css Twitter引导2顶部导航下拉列表，由subnav条覆盖

我正在创建一个Twitter引导主导航和子导航，主导航上有一个下拉菜单。问题是，我永远无法让下拉菜单显示在副导航上方。以下是HTML：

<div class="mainnav navbar navbar-fixed-top navbar-inverse">
    <div class="navbar-inner">
        <div class="container">
            <ul class="nav">
                <li class="dropdown">
                    <a href="#" class="dropdown-toggle" data-toggle="dropdown">
                        Settings
                    </a>
                    <ul class="dropdown-menu">
                        <li><a>test 1</a></li>
                        <li><a>test 2</a></li>
                    </ul>
                </li>
            </ul>
        </div>
    </div>
</div>

<div class="subnav navbar navbar-fixed-top">
    <div class="navbar-inner">
        <div class="container">
            <ul class="nav">
                <li><a>foo</a></li>
                <li><a>bar</a></li>
            </ul>
        </div>
    </div>
</div>

---

​

没关系，这是我自己想出来的

.subnav {
    top: 40px;
    z-index: 1020; // make z-index smaller than .mainnav
}

JSIDLE链接：

尝试一下z索引：

.subnav {
    position: absolute;
    top: 40px;
    z-index: 10;

}

.mainnav .dropdown-menu {
    position: absolute;
    z-index: 20;
}

---

上面是什么意思？z指数？在此，将此作为起点示例：

.subnav {
    position: absolute;
    top: 40px;
    z-index: 10;

}

.mainnav .dropdown-menu {
    position: absolute;
    z-index: 20;
}