JSFiddle - React, Tailwind, and code Playground

HTML

<div id="foo">
    <div id="left"></div>
    <div id="right"></div>
</div>

CSS

#foo {
    display: table;
    //height: 400px;
    position: relative;
    width: 500px;   
}

#foo > div {
    display: table-cell;
    height: 100%;
    position: absolute;
}

#left {
    background: blue;
    left: 0;
    right: 200px;
}

#right {
    background: green;
    right: 0;
    width: 200px;
}