Let's say I have two groups of two indexers, each group in its own cluster. Can I use a single master cluster node to control both clusters, or must I deploy one for each?
In theory, with enough hardware and documentation so you don't confuse future you, you could put two different Splunk installations on a single server, and each installation controls a different cluster... But a single Splunk installation cannot be the cluster master for multiple different clusters. (It can control a single multi-site cluster, but this is rather different than having two separate clusters)
Simply structurally it cannot happen within a single installation first, server.conf is where ultimately you would put your cluster configuration, and there's a single [clustering]
stanza for the cluster master to set its details (SF/RF) for the cluster. See http://docs.splunk.com/Documentation/Splunk/6.5.2/Indexer/Configuremasterwithserverconf and http://docs.splunk.com/Documentation/Splunk/6.5.2/Admin/Serverconf
Furthermore there's the problem of pushing configuration to the slaves which comes from a single $SPLUNK_HOME/etc/master-apps
as well.
So technically you could have a single Node, it's not a single installation on a single Node.
Yes, you can use single Master node
Not true. Each cluster has exactly one master node. The cluster can have any number of peer nodes and search heads
Can you share your background info on this?