PSU Patch on 2 standby database
Solution 1:
There are two parts to patching. 1.software binaries patching the binaries and library files 2. The database objects that rough catbundle or dispatch.
It may not cause any problems directly not patching the database component but there would be some conflicts and issues with functionality.It may also be not supported by oracle. It is advisable to keep primary and all standby databases at the same level.
The database component in the standby database would automatically be updated during the replication.