$tag_string = '';

for ($i = 0; $i < 8; $i++) {
    for ($j = 0; $j < 8; $j++) {

        $tag_to_use = get_post_meta($post->ID, '_tag_to_use_'.$i.'_'.$j, true);
        $tag_string = $tag_to_use ? $tag_string .= $tag_to_use . ',' : $tag_string .= '';

    }
}

if ( substr( $tag_string, -1) == ',' ) {
    $tag_string = rtrim( $tag_string, ',' );
}

echo do_shortcode( '[products tag="' . $tag_string . '"]' );

}

echo do_shortcode( '[products tag="' . $tag_string . '"]' );

echo do_shortcode( '[products tag="test,test1"]' );

$post_meta = get_post_meta($post->ID); // This returns an array with all the data in position 0, if I remember correctly
for ($i = 0; $i < 8; $i++) {
  for ($j = 0; $j < 8; $j++) {
    if ( isset( $post_meta[0]['_tag_to_use_'.$i.'_'.$j] ) ) {
      $tag_to_use = $post_meta[0]['_tag_to_use_'.$i.'_'.$j];
      // The meta value might be in a array with the position 0, var_dump $tag_to_use to see if this is the case
      // $tag_to_use = $post_meta[0]['_tag_to_use_'.$i.'_'.$j][0];
      $tag_string = $tag_to_use ? $tag_string .= $tag_to_use . ',' : $tag_string .= '';
    }
  }
}