display most popular tags in two columns

In my opinion you are doing it so complicated.

I would rather do a classic query and then through css it made into 2-column.

function top_tags() {
    $tags = get_the_tags();

        if($tags)
        {
             $output="<h4>Top tags</h4> <ul class="top_tags">";
             foreach($tags as $tag)
                   $output .= '<li><a href="'.get_tag_link($tag->term_id).'" title="'.$tag->name.'">'.$tag->name.'</a></li>';

             $output .= '</ul>';

             echo $output;
         }
}

and your css

.top_tags li {display:block; float:left; width:50%; clear:right;}