The issue was never resolved. One major roadblock was that we never had a hardware available supporting the second Ethernet controller on RMII.
Would it be an option for you to provide us such a hardware? If yes, I will verify internally how much resources we could put into debugging the problem.
The difference of the module is only in the flash storage and therefore not relevant for the Ethernet issue.
However, the post was for Linux, which has a major impact on how the second instance needs to be activated.