div.left5 {
    float: left;
    width: 50%;
    color: #006;
    background: #fff;
    margin: -3px 0 0 -3px;
    border-width: 0 3px 3px 0;
    border-style: solid;
    border-color: #f00;
    padding: 3px 0 0 3px;
}
div.right5 {
    color: #600;
    background-color: #ccc;
    border: 3px #f00 solid;
}