<style>
  #box{
    display: flex;
    height: 500px;

  }
#box-1 {
    background-color: blue;
    width: 50%;
    height: 50%;
  }

#box-2 {
    background-color: red;
    width: 50%;
    height: 50%;
  }
</style>

<div id="box">
  <div id="box-1"></div>
  <div id="box-2"></div>
</div>

#box-1 {
    background-color: blue;
    width: 50%;
    height: 50%;
    display: flex;
  }

#box-2 {
    background-color: orangered;
    width: 50%;
    height: 50%;
    display: flex;
  }
</style>

<div id="box-1"></div>
<div id="box-2"></div>