JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent">
  <div class="left margin-right-auto">Left</div>
  <div class="right">Right</div>
</div>

<div class="parent">
  <div class="left">Left</div>
  <div class="fill-remaining-space"></div>
  <div class="right">Right</div>
</div>

SCSS

.left {
    padding: 1em;
    background-color: #337ab7;
}

.right {
    padding: 1em;
    background-color: #5cb85c;
}

.parent {
    display: flex;
    overflow: hidden;
    margin: 1em 0;
    color: #fff;
    border: 1px solid #333;
    align-items: center;
}

.margin-right-auto {
   margin-right: auto;
}

.fill-remaining-space {
  flex: auto;
}