How do you find out what resorts are affiliated with both RCI and II? Are there any?
If you Google, RCI resort directory or Interval International resort directory, it will bring you there. They're listed on the home page -- II is under the Explore & Plan link. You don't have to sign in to get there. From there, you can compare. Sounds like a daunting task. You will need an ownership to join either one.
Kahana Beach in Maui, Water's Edge in CT, some of the Wyndhams have dual affiliation even though Wyndham owns RCI, I think Riverside suites in San Antonio goes both ways. Villas of Sedona, too, so probably Sedona Springs, as well. I am sure there are a lot of them.
Suggestions: buy on the secondary market, look for low maintenance fees because you will be paying them every year, look for an area not swamped with timeshares but a desirable place to travel to at time people want to travel there.
We have dual membership and have one resort listed with both but also own other single affiliation resorts with both companies.