Interesting challenge! If you worked with the same mentee but two different organizational venues, you can show this as a cumulative role by listing the dates and the role, along with both organizations. It's kind of a technical/typographical challenge, but your goal here is to be accurate (you have worked consistently for an extended period with this person) as well as readable (a really complex resume bullet is hard to read, and might be ignored.)
So put yourself in the admission reader's shoes, and try to figure out how you could simply and accurate deliver the information your reader is looking for.

Good luck!
Margaret Strother