Using more than one 'meta_compare' in query vars

I’m trying to filter my query with a custom post type setting the following query vars:

$wp_query->query_vars['meta_key'] = 'vence';
$wp_query->query_vars['meta_compare'] = '<';
$wp_query->query_vars['meta_value'] = date('Ymd', current_time('timestamp'));

This works perfectly. My problem is that I would like to add a second set of meta_keys to compare.

Does anyone know how to do this?

Moises