Others far more knowledgeable than I about this will chime in, but my one personal experience is with Hawaiian Airlines and Aloha Airlines.
If you have a Hawaiian Airlines Visa, you can pool miles. These miles can be used on HA's partners; or, conversely, you can credit miles on your HA partner flights to HA. You can check out their partners at
www.hawwaiianair.com
Aloha Airlines also shared miles. But it is really only useful, afaik, for Aloha flights. While you can get United miles credit flying Aloha, I don't think the reverse possiblity exists.