xbor

.contain {
    display: inline-block;
    vertical-align: top;
}
.contain:first-of-type {
    margin-left: calc((100% - 1200px) / 2);
}
.contain + .contain {
    margin-left: 30px;
}
:not(aside).contain {
    width: 900px;
}
aside.contain {
    width: 270px;
}

<aside class="contain"><aside/>
    <div class="contain"><div/>