SOURCE

<div class="box">
  <div class="column">
    <span class="item">11</span>
    <span class="item">12</span>
  </div>
  <div class="column">
    <span class="item">21</span>
    <span class="item">22</span>
  </div>
</div>

.box {
  display: flex;
  flex-wrap: wrap;
  align-content: space-between;
}

.column {
  flex-basis: 100%;
  display: flex;
  justify-content: space-between;
}