How to split a loop into two columns

You will need to create css classes for a left and right column (you can use bootstrap if you like) after creating the classes try this

<div class="headlines" data-interval="<?php echo ($interval>0)?$interval*1000:'false'; ?>">
    <ul>
    <?php  
        $i = 0;
        while ( $r->have_posts() ) {
            $r->the_post();
        if($i % 2 == 0){ //even line
    ?>
        <li class="left-col"><a href="https://wordpress.stackexchange.com/questions/191730/<?php the_permalink(); ?>"><?php the_title(); ?></a><span class="time_diff"><?php echo ' - ' . dw_human_time_diff( get_the_time('U'), current_time('timestamp') ); ?></span></li>
    <?php } else { ?>
        <li class="right-col"><a href="https://wordpress.stackexchange.com/questions/191730/<?php the_permalink(); ?>"><?php the_title(); ?></a><span class="time_diff"><?php echo ' - ' . dw_human_time_diff( get_the_time('U'), current_time('timestamp') ); ?></span></li>
    <?php };
      $i++; }
    ?>
    </ul>
</div>