How to create meta fields with more than 1 relation?

A better approach would be to save two meta fields. One for the candidateID and one for the recruiter.

Then when you need to query the posts you should use the WP_Meta_Query with both meta fields and the relation argument set to ‘AND’:

$args = array(
  'post_type'  => 'opening',
  'meta_query' => array(
    'relation' => 'AND',
    array(
      'key'     => 'open_candidate',
      'value'   => $candidateID,
      'compare' => '='
    ),
    array(
      'key'     => 'recruiter',
      'value'   => $recruiterID,
      'compare' => '='
    )
  )
);

$posts = new WP_Query( $args );

Or if you have the post and need to find the candidate and the recruiter, you can just use:

$candidateID = get_post_meta( $post_id, 'open_candidate', true );
$recruiter = get_post_meta( $post_id, 'recruiter', true );

This hasn’t been tested but it should work.

Codex reference for WP_Meta_Query: https://codex.wordpress.org/Class_Reference/WP_Meta_Query