My understanding is that only kbin can interact with both Lemmy and Mastodon while also showing its own threads/magazines/whatever. Lemmy can only see Lemmy and Mastadon can only see Mastodon.
I'm on kbin and also have access to read and comment on Lemmy posts, though I'm not sure how to access Mastadon as I don't see them show up when I search for other communities..
Edit: looks like I have to search Microblogs when I want to see Mastadon. Seems kind of confusing though, I don't think I understand how Mastodon works.