<div class="flex">
  <div class="item item-a"></div>
  <div class="item item-b"></div>
</div>
.flex {
  display: flex;
  flex-wrap: wrap;
  
  > * {
    height: 120px;
    font-weight: 700;
    font-family: arial, sans-serif;
    color: #222;
  }
}
/* コンテナーのブレイクポイント
  320px * 2 = 640px */
.item {
  box-sizing: border-box;
  flex: 1 0 calc(100% - 320px);
}
.item-a {
  background: #faa;
}
.item-b {
  background: #aea;
}

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