Is it possible to make .item-1 height flexible and adjust .item-2 height? For example:
- if .item-1height is10%then.item-2height is90%
- if .item-1height is11%then.item-2height is89%
So depending on the content of the .item-1 we should resize it.
HTML
<div class="container">
    <div class="item item-1">1</div>
    <div class="item item-2">2</div>
</div>
CSS
html,
body,
.container {
    height: 100%;
}
.container {
    display: -webkit-box;
    display: -moz-box;
    display: -ms-flexbox;
    display: -webkit-flex;
    display: flex;
    -webkit-flex-flow: column;
    -moz-flex-flow: column;
    -ms-flex-flow: column;
    flex-flow: column;
}
.item {
    background: #eee;
    margin: 1px;
}
.item-1 {
  -webkit-box-flex: 3 3 100%;
  -moz-box-flex: 3 3 100%;
  -webkit-flex: 3 3 100%;
  -ms-flex: 3 3 100%;
  flex: 3 3 100%;
}
.item-2 {
  -webkit-box-flex: 1 1 100%;
  -moz-box-flex: 1 1 100%;
  -webkit-flex: 1 1 100%;
  -ms-flex: 1 1 100%;
  flex: 1 1 100%;
}
JSFiddle: http://jsfiddle.net/vpetrychuk/SYZtg/2
 
    