I have found two collaborative pen name author credits:
Pittacus Lore: https://bookbrainz.org/author/639e53b5-efb1-493b-ad13-9d629f932413
James S. A. Corey: https://bookbrainz.org/author/c5c71242-f54c-4327-9510-a81064a9a852
They are listed with the Type as “Group”.
I don’t know if this is correct, or whether these names should only be listed as Aliases in the respective individual’s author credit.
There is also Erin Hunter: https://bookbrainz.org/author/23dbe270-f61e-4a41-abfe-da5938f0cd27
This credit treats the individuals as aliases and has a pen name relationship to one of the writers: Inbali Iserles