I worked it all out. Here is my solution.
<?php if ( get_field('primary_and_secondary_selector') ) : ?>
<?php while ( has_sub_field('primary_and_secondary_selector') ) : ?>
<div class="grid_3">
<?php $primary_queries = get_sub_field('primary'); ?>
<?php if ( $primary_queries ) : ?>
<?php foreach( $primary_queries as $primary_query ) : ?>
<?php echo wp_get_attachment_image(get_field('supplementary_logo', $primary_query->ID), 'medium' ); ?>
<?php endforeach; ?>
<?php wp_reset_postdata(); ?>
<?php endif;?>
<?php $secondary_queries = get_sub_field('secondary'); ?>
<?php if ( $secondary_queries ) : ?>
<ul>
<?php foreach( $secondary_queries as $secondary_query ) : ?>
<li><?php echo wp_get_attachment_image(get_field('supplementary_logo', $secondary_query->ID), 'medium' ); ?></li>
<?php endforeach; ?>
</ul>
<?php wp_reset_postdata(); ?>
<?php endif;?>
</div>
<?php endwhile; ?>
<?php endif;?>
