<section id="box_parent">
  <section id="box_child_a">
    子どもA
  </section>
 
  <section id="box_child_b">
    子どもB
  </section>
</section>
#box_parent{
    display: flex;
    justify-content: space-between;
    flex-wrap: wrap;
    width: auto;
}

#box_child_a{
    width: 48%;
    background-color: #aaa;
}
#box_child_b{
    width: 48%;
    background-color: #bbb;
}
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