<div class="box">
  <div class="box__red"></div>
  <div class="box__green"></div>
</div>

.box {
  display: flex;
  flex-direction: row;
}
.box__green, .box__red {
  width: 50%;
  height: 50px;
}

.box__green {
  background-color: green;
}

.box__red {
  background-color: red;
}