Sample index.php file to show how you could create a new row after two posts.

index.php

<?php
/**
 * Sample index.php file for displaying two posts per row
 */

get_header(); ?>

    <!-- Row for main content area -->
    <div id="content" class="row">

        <div id="main" class="twelve columns" role="main">

            <div class="post-box">

            <?php if ( have_posts() ) : ?>

                <?php

                    (int) $posts_counter = 1; // Initiate custom counter
                    while ( have_posts() ) : the_post();

                    if ( $posts_counter % 2 == 1 ) {
                        echo '<div class="row">'; // draw the opening div.row
                    }
                ?>
                    <div class="six columns">
                        <?php get_template_part( 'content', get_post_format() ); ?>
                    </div>
                <?php
                    // Make sure we close the row after the last post anyways.
                    if ( $posts_counter % 2 == 0 || $posts_counter == $wp_query->post_count ) {
                        echo '</div>'; // draw the closing div.row
                    }
                    $posts_counter++; // +1 for the counter
                    endwhile;
                ?>

            <?php else : ?>
                <?php get_template_part( 'content', 'none' ); ?>
            <?php endif; ?>

            <?php if ( function_exists( 'required_pagination' ) ) {
                required_pagination();
            } ?>
            </div>

        </div><!-- /#main -->

    </div><!-- End Content row -->

<?php get_footer(); ?>