I solved it like this. Although if there are more than 3 co-authors problems could arise
$tagx = get_the_terms($post->ID , 'autoria');
$number = count($tagx);
if ( $number == 1) {
$first_author = $tagx[0];
echo '<strong>'; echo esc_html( $first_author->name ); echo '</strong>';
}
if ($number == 2) {
$first_author = $tagx[0];
$second_author = $tagx[1];
echo '<strong>'; echo esc_html( $first_author->name ); echo '</strong>,';
echo ' <strong>';
echo explode(' ', $second_author->name)[3] . ' ';
echo explode(' ', $second_author->name)[2] . ' ';
echo explode(' ', $second_author->name)[1] . ' ';
echo explode(' ', $second_author->name)[0] . ' ';
echo '</strong> ';
}
if ( $number > 2) {
$first_author = $tagx[0];
echo '<strong>'; echo esc_html( $first_author->name ); echo '</strong>, ';
foreach ($tagx as $tag) {
$tag_array[] = $tag->name;
if ($tag_array[1]) {
echo ', <strong>';
echo explode(' ', $tag->name)[3] . ' ';
echo explode(' ', $tag->name)[2] . ' ';
echo explode(' ', $tag->name)[1] . ' ';
echo explode(' ', $tag->name)[0] . ' ';
echo '</strong> ';
}
if ($tag_array[2]) {
echo 'y <strong>';
echo explode(' ', $tag->name)[3] . ' ';
echo explode(' ', $tag->name)[2] . ' ';
echo explode(' ', $tag->name)[1] . ' ';
echo explode(' ', $tag->name)[0] . ' ';
echo '</strong> ';
}
}
}