Setting same persistent_id for you MemCache server is a normal case when
you want a pool of servers.
Example from the doc :
/* Create a persistent instance */
$m2 = new Memcached('story_pool');
$m3 = new Memcached('story_pool');
/* now $m2 and $m3 share the same connection */
Note that even using persistent id you must re-add your servers, see http://php.net/manual/en/memcached.construct.php#93536
See also this important note : http://php.net/manual/en/memcached.construct.php#106865
Your requests are independent. PHP doesn't share its requests. So If
you've 2 (or more) requests with the same persistent_id both will live into
their own process.