2 servers with 2 local memcached: flush both
I suggest you to use Drush aliases.

Here is the quote from the documentation http://drush.org/en/master/usage/#site-aliases


  Drush lets you run commands on a remote server, or even on a set of remote servers. Once defined, aliases can be references with the @ nomenclature


Also from here https://raw.githubusercontent.com/drush-ops/drush/master/examples/example.aliases.drushrc.php


  Alias groups (aliases stored together in files called GROUPNAME.aliases.drushrc.php, as mentioned above) also create an implicit namespace that is named after the group name.


Finally, these are few simple steps to achieve what you want:


Create production.aliases.drushrc.php file
Put in this file aliases for both Drupal 1 and 2
Then you can clear cache on all site at once with drush @production cc all