split2

<style type="text/css">
section {
    width: 800px;
    display: flex;
}
div.left, div.right {
    outline: solid 1px #ccc;
    overflow: auto;
}
div.left {
    width: 20%;
    resize: both;
}
div.right {
    flex: 1 1 0;
}
</style>

<section>
    <div class="left"> 
        Lorem ipsum dolor sit amet, consectetuer adipiscing elit.
    </div> 
    <div class="right"> 
        Lorem ipsum dolor sit amet, consectetuer adipiscing elit. 
    </div>
</section>