Flex 2 columns - 33% x 66%

<!DOCTYPE html>
<html>
<head>
<style>
.flexContainer {
  display: flex;
  flex-direction: row;
  background-color: #cdcdcd;
}

.flexColumn {
  background-color: #FFFFFF;
}

#leftColumn{
    width:33%;
    margin: 0.4em 0.2em 0.4em 0.4em;
    overflow: auto;
}

#rightColumn{
    width:66%;
    margin: 0.4em 0.4em 0.4em 0.2em;
}

</style>
</head>
<body>

<div class="flexContainer">
  <div id="leftColumn" class="flexColumn">1</div>
  <div id="rightColumn" class="flexColumn">2</div>
</div>

</body>
</html>