Do like this:
HTML
<div class="parent">
<div class="child"></div>
<div class="child"></div>
</div>
CSS
.parent{
width: 400px;
background: red;
}
.child{
float: left;
width:200px;
background:green;
height: 100px;
}
This is working jsfiddle. Change child width
to more then 200px
and they will stack.