What Oracle says about this combination:
Oracle Data Guard and Real Application Clusters (RAC) are complementary to each other.
RAC addresses system failures. It provides rapid and automatic recovery from failures that do not affect data - such as node failures, or instance crashes. It also provides increased scalability for an application.
Data Guard, on the other hand, provides data protection through the use of transactional consistent primary and standby databases, which neither share disks nor run in lock step. This enables recovery from site disasters or data corruptions.
Both Data Guard Redo Apply (physical standby database) and Data Guard SQL Apply (logical standby database) can be used with RAC.
Customers should use a combination of Oracle Data Guard and Real Application Clusters to get the benefits of both data-level and system-level protection, resulting in an end-to-end High Availability solution. Please refer to Oracle's Maximum Availability Architecture framework for best practice recommendations on integrating Oracle Data Guard and Oracle Real Application Clusters for a highly available architecture.