JSFiddle - React, Tailwind, and code Playground

HTML

<div class="flex-box">
    <div class="flex-item"></div>        
    <div class="flex-item"></div>
</div>

CSS

.flex-box {
    display: flex;
    height: 300px;    
    background: gray;
}

.flex-item {
    flex: 1;
    border-right: 2px solid #000;
    background: green;
}