HTML

flex 2 columns per row

.item {
  width: 100%
}

.container {
  display: flex;
  flex-wrap: wrap;
}

.container > div {
  flex: 50%; /* or - flex: 0 50% - or - flex-basis: 50% - */
  /*demo*/
  box-shadow: 0 0 0 1px black;
  margin-bottom: 10px;
}
/////////////html
<div class="container">
  <div class="item">1</div>
  <div class="item">2</div>
  <div class="item">3</div>
  <div class="item">4</div>
</div>