Here is a proposal to add support for loading an identity's attributes from multiple security realms: -

https://github.com/wildfly/wildfly-proposals/pull/205

This is being handled under the following Jira issues: -

https://issues.jboss.org/browse/ELY-1804
https://issues.jboss.org/browse/WFCORE-4447
https://issues.jboss.org/browse/WFLY-12051

Regards,
Darran Lofthouse.