If your building an 24*7 Portal environment, or environment that can be brought down even for maintenance or upgrade then you will have to use the Golden Architecture defined by IBM.
IBM's Golden architecture defines that you should have two portal servers or two portal clusters, when you bring down one of the clusters for maintenance other takes care of serving user request.
This is how you can setup the database for Golden architecture.
As you can see each of the cluster would get its own release database but they will share customization and community databases with other clusters in the production.
If your clusters are in different location then you only have to replicate community and customization database between them.