Query_post($args)

Refer to multiple custom field handling in WP_Query:

$args = array(
    'meta_query' => array(
        'relation' => 'OR',
        array(
            'key' => 'city_1',
            'value' => array('NY','LA'),
            'compare' => 'IN'
        ),
        array(
            'key' => 'city_2',
            'value' => array('NY','LA'),
            'compare' => 'IN'
        )
    )
);
query_posts($args);