Seen a few Courtyard/RI that share the same building in major cities.

Specifically stayed at the Courtyard Central Park before and it shares the same lobby space but with different elevators from the RI. Presumably same management company and targeting long term or family travelers (RI) vs. more business travelers (CY) from a booking standpoint. Amenities and everything else virtually all the same -- I've always picked the CY side purely due to better earnings and when traveling solo I don't need any of the things delivered under the RI brand standards.

I believe Austin has one of these too.

I think Hilton does the same thing as well combining HGI and Homewood Suites in some areas.

I've also stayed at the Hilton Garden Inn Charlotte Uptown which shares a parking facility with the Hampton Inn but nothing else AFAIK (no shared lobby at least during my stay and no other obvious shared amenities albeit it was a brief stay).

The other obvious one that comes to mind is Hyatt Ziva and Hyatt Zilara -- they often go together (e.g., Cancun) with some shared facilities between the two with obviously one being Adult only and the other being Adult + Kid friendly