Flexbox with a column on multiple rows

HTML

<div class="container">
    <div class="parent last">
    <div class="el"></div>
    </div>
    <div class="parent first">
        <div class="flexbox">
            <div class="el"></div>
            <div class="el"></div>
        </div>
    </div>    
</div>

CSS

.container {
    font-size: 0;
}
.parent {
    display: inline-block;
    vertical-align: top;
}
.parent.first {
    width: 50%;
}
.parent.last {
    width: 50%;
}

.flexbox {
    display: flex;
    flex-flow: column wrap;
}
.flexbox .el:nth-child(1) { flex: 1; }
.flexbox .el:nth-child(2) { flex: 2; }

.el {
    height: 100px;
    background-color: red;
    margin: 5px;
}
.last .el {
    height: 210px;
    background-color: blue;
    margin: 5px;
}