loop.php

$artists = Timber::get_posts($artist_args);
foreach( $artists as $artist ) {
	$connected = new WP_Query( array(
        	'connected_type' => 'portfolios_to_artists',
        	'connected_items' => $artist,
		'nopaging' => true
    	) );
	$artist->portfolios = Timber::get_posts($connected);
}
$data['artists'] = $artists;
Timber::render('loop.twig', $data);

loop.twig

{% for a in artists %}
%a.artist-card{:href => "{{a.link}}", :id => "artist-{{a.ID}}"}
  {% for p in a.portfolios %}
  	Portfolio {{loop.index}}
  {% endfor %}
  {{ a.slug }}
  %h3 {{ a.title }}
  {% if a.get_field('artist_bio') %}
  {% endif %}
{% endfor %}