<html>
<head>
<style>
div {
  box-orient: vertical;
  -moz-box-orient: vertical;
  -webkit-box-orient: vertical;
  display: -moz-box;
  display: -webkit-box;
  display: box;
  background-color: blue;
  width: 100px;
  height: 100px;
}

.first {
  visibility: hidden;
  -moz-box-flex: 1;
  -webkit-box-flex: 1;
  box-flex: 1;
  background-color: red;
  width: 50px;
  height: 100px;
}

.second {
  background-color: green;
  -moz-box-flex: 1;
  -webkit-box-flex: 1;
  box-flex: 1;
  width: 50px;
  height: 100px;
}

</style>
</head>
<body>
<div>
  <div class="first"></div>
  <div class="second"></div>
</div>
</body>
</html>
