Edit in JSFiddle

.col-2 {
    float: left;
    width: 50%;
    padding-left: 10px;
    padding-right: 10px;
    background-color: #ffdbdb;
}
<section class="contents">
    <div class="col-2">첫번째 단</div>
    <div class="col-2">두번째 단</div>
</section>