The Mbin software federates with both Lemmy and Mastodon at once. The Interstellar app works for it (+ also PieFed, though the latter does not currently federate with Mastodon, only Lemmy and itself). The largest Mbin instance iirc is fedia.io. By default you see the "Threads" content shared with Lemmy, but clicking the top left icon and then choosing "Microblog" will show content from Mastodon.
The terminology can be quite confusing, especially on Mbin compared to Lemmy - e.g. Lemmy-style "Communities" on Mbin are called "Magazines". The sharing between Lemmy, Mbin Threads/Magazines, and PieFed is called the "Threadiverse", while the sharing between Mastodon and Mbin Microblog/People does not have a unique name iirc (though I don't use Mbin or any Mastodon or Microblogging so apologies if I am incorrect here), so it just uses the wider name "Fediverse", which also includes the Threadiverse inside of it as well. A wiki is in progress, looking for contributors. That community !fediverse@piefed.social has a ton of great reading material, as too does !Fediverse@lemmy.world.
