Real Application Clusters, for the most part, does not care if you have one application or two stored in a database. Just store the application data in separate schemas. One working theory is to have one application's users connect to NodeA, another application's users connect to NodeB, and so on. In case of a failure of a node, the application users fail over to another node in the grid.
With Oracle 10g, you can perform the patchset application in a rolling upgrade fashion. Check the patchset documention for more information.
This was first published in December 2005