Edit in JSFiddle

<div class="container">
    <div class="box left">This is some filler text for this example</div>
    <div class="box right">This is some filler text for this example</div>
</div>

.container {
    width: 300px;    
    outline: 1px solid red;
}

.container:after {
    content: "";
    display: table;
    clear: both;
}

.box {
    width: 50%;    
    outline: 1px solid blue;       
}

.left {
    float: left;
}
.right {
    float: right;
}