Sorry for the very slow reply.
From what you wrote, I think you want something analogous to the following:
1) Four servers A, B, C, D that for SSO purposes are in one partition.
2) For HttpSession replication purposes A and B are in a partition and C and D are in a different partition.
To do this:
1) Edit the tc5-cluster-service.xml file on C and D and change the multicast address or port so they're different from the version of the file you have on A and B. Now A/B will be one partition, C/D another.
2) Make a copy of tc5-cluster-service.xml; call it sso-cluster-service.xml.
3) Edit the multicast port and address so they are different from those used in either of the tc5-cluster-service.xml files.
4) In the top of the file, where it says
<mbean code="org.jboss.cache.TreeCache" name="jboss.cache:service=TomcatClusteringCache">
change it to
<mbean code="org.jboss.cache.TreeCache" name="jboss.cache:service=SSOClusteringCache">
5) Put the sso-cluster-service.xml file in the deploy dir of all 4 servers. Now you have a TreeCache that is shared across all 4 servers.
6) Edit the server.xml file in each server to add the following attribute to the ClusteredSingleSignOn's element:
Now ClusteredSSO will use the 4-server TreeCache rather than either of the 2-server caches.