You're welcome. I'm glad you like it. I'm not sure I understand what you mean with your custom field name... the sub title is already stored in a meta field. You can retrieve the subtitle information with the get_the_subtitle() function. I believe that Yoast's SEO plugin filters wp_title(). So you could also filter wp_title() and pass in the value from the subtitle field.
Just as a suggestion, you could perhaps try:
$subtitle = get_the_subtitle( $post->ID );
$title .= " - " . $subtitle;
add_filter('wp_title','kia_add_subtitle_to_yoast', 99 );
It is totally untested, but might work. Unfortunately I can't really support the SEO plugin. Best of luck.