Starting from WPS 6.0.1, you can architect multiple portal clusters within the same WebSphere Cell.
Please note that multiple cluster architecture has one very important limitation, which is that, only a single J2EE enterprise application of a unique name can be deployed into a given websphere cell at any one time. All enterprise applications are cell scoped. This limitation makes it impossible to deploy different versions of the same enterprise application against the different portal clusters within the same cell, as the case might be during the periods of 24*7 maintenance. This extends to the WebSphere Portal server itself, which consist of number of enterprise applications that make up the effective runtime and also to the every portlet application deployed within the solution. As a consequence, it is not possible to upgrade one portal cluster in isolation from another as the underlying enterprise application and supporting class libraries are common to both.
This architecture meets requirement of continuous operation during scheduled or unscheduled architecture but it is not a good feet if you want truly 24*7 service. In case of requirement for 24*7 service you should think about WebSphere Portal Server Gold Availability standard