Hi @toniup
Within your loop, you could use the get_post_type() to see which post type is the post is.
You could then add a conditional. Something like:
<?php
$posts = get_field( 'relazioni_strutture' );
if ( $posts ) : ?>
<?php foreach( $posts as $post) : ?>
<?php setup_postdata( $post ); ?>
<?php $post_type = get_post_type();
if( $post_type == 'Struttura' ):
?>
<?php the_title(); ?>
<?php endif; #$post_type ?>
<?php endforeach; ?>
<?php wp_reset_postdata(); ?>
<?php endif; ?>
May need tweaking but should get you underway
