<div class="wrapper">
  <div class="box1">123</div>
  <div class="box2"></div>
</div>

.wrapper {
  display: flex;
  align-items: stretch;
}

.box1 {
  width: 50px;
  height: 100%;
  background-color: green;
}

.box2 {
  width: 50px;
  height: 100px;
  background-color: blue;
}