JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
  <div class="elem1">
    All you need is love!
  </div>
  <div class="elem2">
    <div class="big-box">
    
    </div>
  </div>
</div>

CSS

.container {
  display: flex;
  width: 200px;
  border: 1px solid blue;
}

.elem1 {
  flex: 1 1 0px;
  border: 1px solid red;
}

.elem2 {
  flex: 1 1 0px;
  border: 1px solid red;
}

.big-box {
  width: 250px;
  height: 20px;
  background: orange;
}