In OBI 11g, you could achieve the clustering in two ways as follows1. Horizontal Scaleout: You set up a new system/instance/physical machine and cluster this with the primary one.2. Vertical scaleout: Increasing the number of BIServer/sawserver/javahostserver/.. instances in the same box. You could multiple nqserver processes running in this case.So, based on our load/architecture/need you could go for any of it. You could refer to http://docs.oracle.com/cd/E23943_01/doc.1111/e15722/bi_clusters.htm#CIHJEHCH for more information on this.
See the section named "What happens if Adminstration server fails" in the below link:
http://docs.oracle.com/cd/E14571_01/apirefs.1111/e13952/taskhelp/startstop/StartServersInADMIN.html (Start ManagedServer in Admin Mode).
The above concepts are for core Weblogic Server Cluster Domain. Since OBIEE is on top of this same Weblogic server, the above rules should apply mostly.
I hope you already saw these links. If not, its worth to read it once