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. The port specified must be open between machines as this will be used for the replication messages.

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>