How to share Redis cache across Drupal servers?
If you use the Redis module for Drupal, you can set the database and prefix used by your sites. The default Redis database is 0 (so multiple sites use the same database) but the default prefix is the HTTP_HOST variable (so multiple sites use different prefix). You can however, set a shared prefix on both site in their settings.php:

$conf['cache_prefix']['default'] = 'mysite_';


You can also set prefix per cache bin, to only share a subset of the cache between them. See Redis module's README.txt for more details.

Of course, this should only be done if your are sure the cache can actually be shared. In your case, I guess the sites share the database and are used to balance the load.