By stacking LoginModules it was possible using PicketBox to attempt to authenticate using one remote store and if that failed try the next store in the list.
This RFE is to consider the use case where identities could be located across multiple stores and how they are aggregated together.
Additionally this use case should consider how the authorization information could be loaded from multiple sources and merged.
This RFE is not about fail over in the event of a realm being unavailable although it may be related.
This RFE is created as a result of comparing the differences between the PicketBox JAAS architecture and the Elytron architecture so I would not recommend this proceeds without some real world use cases identified.