.equal-height-container {
  max-width: 900px;
  margin: 0 auto;
  display: flex;
}

.first {
  background-color: #FFF;
  padding: 20px;
  flex: 1;
}

.second {
  background-color: yellow;
  flex: 1;
  display: flex;
  flex-direction: column;
}

.second-a {
  background-color: #c0dbe2;
  flex: 1;
}

.second-b {
  background-color: #cdf1c3;
  flex: 1;
}