Thank you for posting this in Microsoft Q&A.
Currently this is a known behavior. As part of SSPR configuration you can only enable one Microsoft Entra group for SSPR using the Microsoft Entra admin center. As part of a wider deployment of SSPR, Microsoft Entra ID supports nested groups.
However, as mentioned above SSPR configuration supports nested groups. This means you can make the 2nd group as part of first group so that even second group will be configured for SSPR.
You can refer below public article for this,
https://learn.microsoft.com/en-us/entra/identity/authentication/tutorial-enable-sspr
If you are looking for functionality to add second group separately then you can submit feedback in Azure feedback portal.
https://feedback.azure.com/d365community/forum/22920db1-ad25-ec11-b6e6-000d3a4f0789
Let me know if you have any further questions.
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