Kbin combines Lemmy and Mastodon right out of the box - no setup needed.

Kbin is new and undergoing development, so the documentation is lacking, but it's pretty easy to use. It combines posts from the Fediverse (including lemmy) under its Threads section, and posts from Mastodon and other similar federated services in its Microblog section.

It also allows you to post in both formats, which is pretty fun. It also has search capacity for tags across instances (kbin.social/tag/cat) and domains (kbin.social/d/lemmy.world), the latter of which allows you to subscribe to or block those domains as you chose. I've posted more detail on a couple of threads here and here.