display:table

<style>
*{ margin:0; padding:0;}
.box{ display:table; width:100%;table-layout:fixed;}
.box div{ display:table-cell; border-bottom:solid 2px #ccc; text-align:center}
</style>

<div class="box">
    <div>tab1</div>
    <div>tab2</div>
</div>