<style>
    * {
        border: 1px solid black;
    }

    .grid-container {
        display: grid;
        grid-template-columns: repeat(12, minmax(0, 5fr));
    }

    .a {
        grid-column-end: span 8;
    }

    .b {
        grid-column-end: span 4;
    }
</style><div class="grid-container"><div class="a">1</div><div class="b">2</div></div>