Session replication only functions for Enterprise Licenses. The servers participating in the replication must have a manual setting configuration done in their prefs.xml file.

Edit the prefs.xml file and fill in the entry for: replicate_session_host_port

Example for the first machine:
<replicate_session_host_port>192.168.1.19:7800</replicate_session_host_port>

and on the second machine a reference back to the first machine:
<replicate_session_host_port>192.168.1.18:7800</replicate_session_host_port>

In the car of multiple machines, each machine references the other machines with a comma separated list. DNS names are fine here as well:
<replicate_session_host_port>machine1:7800,machine3:7800</replicate_session_host_port>

To enable user replication from changes made in the user manager, edit the prefs.xml file and set this flag to true.
<replicated_users_sync>true</replicated_users_sync>

In some special cases you may want the other functionality without the overhead of all session information being replicated between machines. In this case a user would have to re-login if they were suddenly redirected to the other machine...but it avoid wasted resources from replicating all details about a user between machines. Set this to false.
<replicate_sessions>false</replicate_sessions>

If you're using the "S3Crush" mode for users, there is a flag to replicate this config between servers too.
<s3crush_replicated>true</s3crush_replicated>

Job configurations can be replicated too:
<replicate_jobs>true</replicate_jobs>