JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent">
  <div class="button-container">
    <button class="primary">Bouton centré avec test plutôt long pour s'en rendre compte</button>
    <button class="secondary">Bouton à coté</button>
  </div>
</div>

CSS

body {
  display: flex;
  justify-content: center;
}

.parent {
  background-color: #eee;
  height: 300px;
  width: 60%;
  
  display: flex;
  justify-content: center;
  align-items: center;
}

.button-container {
  background-color: #ccc;
  position: relative;
  display: flex;
  flex-direction: row;
  justify-content: flex-end;
  align-items: center;
}

.primary {
  background-color: #55dd77;
  border: none;
  height: 2rem;
}

.secondary {
  background: #ccc;
  border: none;
  text-decoration: underline;
  position: absolute;
  left: 110%;
  white-space: nowrap;
}