Split single-review.php to two parts

Yes, use an if statement, inside your post loop do something like this:

while ( have_posts() {
    the_post();
    $value = get_post_meta( $post->ID, 'metakey', true );
    if ( $value == 'a' ) {
        get_template_part( 'something' );
    } else {
        get_template_part( 'somethingelse');
    }
}

If I were to take your question more literally however, I might interpret it as:

WordPress loads single-review.php when I load my review post, but i want it to load a different file instead if a custom meta has a certain value.

To which there is an answer that would let you do that. But it’s massive overkill for what you’re trying to do, and digging a hole for yourself, especially when such a simple basic alternative exists. I think you have to question why it is that you need to choose a completely different template based on a custom meta, or if it’s the entire template that needs changing