$newline_credits can include any HTML. As you already have it setup for Contact US/Sitemap, you need to add <br />
in front of existing code, something like:
add_filter('tc_credits_display', 'my_custom_credits');
function my_custom_credits(){
$newline_credits = '<br /><a href="<YOURLINK HERE>">Contact Us</a> | <a href="<YOURLINK HERE>">Sitemap</a>';
return '
<div class="span4 credits">
<p> © '.esc_attr( date( 'Y' ) ).' <a href="'.esc_url( home_url() ).'" title="'.esc_attr(get_bloginfo()).'" rel="bookmark">'.esc_attr(get_bloginfo()).'</a>'.($newline_credits ? '<br />'.$newline_credits.'' : '').'</p> </div>';
}