Kbin already federates with Mastodon. It's not a single feed, though, they're on separate tabs. Kbin currently works okay, but per the devs it's an early beta so there's more polish and features to come.
For the record, I'm not sure that combining the two platforms in one feed would be desirable for most users. It seems to me that Lemmy and Mastodon serve different purposes: Mastodon is for broadcasting your opinions to mostly-passive followers, Lemmy is oriented toward conversation.
I think feed separation would be a necessity for sure. It seems like the sort of thing that would fall to devs of 3rd party apps and the like to support
Defederation is the only way 🏁