HTML：

<div id="top">top</div>
<div id="bottom">bottom</div>

CSS：

#top,#bottom {
    position: fixed;
    left: 0;
    right: 0;
    height: 50%;
}
 
#top {
    top: 0;
    background-color: orange;
}
 
#bottom {
    bottom: 0;
    background-color: green;
}