There is little to no documentation that I could find around using multiple Identity Repositories in the same realm.
This is a configuration that is sometimes used but can create issues.
The main issue occurs when writing info against users. OpenAM will try to write the info in each of the defined stores. That is clearly an issue performance-wise, and it will fail if a user is not present in one of the datastores.
I suggest there should also be a discussion within engineering to determine if that configuration should be avoided whenever possible, or at least highlight the context it which it is OK to be used.