I kind of think that’s what kbin is going for

Kbin is probably the closest thing to this. There’s a magazine section for lemmy/kbin communities and there’s another section for microblogging which brings in mastodon posts.

No, there isn’t, and it would be awesome if someone tried.

Both platfroms have a number of mobile apps now … it’d be cool if someone tried something a little new and tried to bring the platforms together.

Others are mentioning kbin here, and while kbin integrates with mastodon better than lemmy, it doesn’t achieve a true fusion, it’s still very much a community-first platfrom (AFAICT).