Adding multiple ganglia clusters is as simple as a new line in your config file.  For OSG installs, the config file you must edit is:

${MonaLisa_HOME}/Service/VDTFarm/vdtFarm.conf

Add your Ganglia server names, one per line, as follows:

*<Cluster Name>{monIGangliaTCP, <hostname>, <port>, }%30

Here's our configuration, which has three different clusters:

*PN_CMS_nodes{IGangliaTCP, localhost, 8649, }%30
*Storage Cluster{IGangliaTCP, thpc-1, 8649, }%30
*ITB Cluster{IGangliaTCP, alliance, 8649, }%30

Notice that we do not use the standard monIGangliaTCP for our module; we use a hacked Ganglia module which reports disk free information.