custom field to always to .get_the_title()?

<?php
function add_subtitle($title, $id) {
    $subtitle = get_post_meta($id, 'myCustomField', true);
    $new_title = $title;
    if(!empty($subtitle))
        $new_title = $subtitle . ': ' . $new_title;
    return $new_title;
}
add_filter('the_title', 'add_subtitle', 10, 2);

Basically, this uses the the_title filter to add the subtitle to your title. It only adds it if that custom field is available, otherwise, it leaves the title alone.