I believe the case with Reaper is: it's not able to see the new audio interface unless you re-open the Audio device settings. At least that's what I always see. Actually I believe that's a bug and that list should be refreshed automatically (like in competitors DAWs), but it's not a big deal to close preferences and open them again.
P.S. Aggregated devices add a lot of latency, so I won't recommend this path.
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