Kbin is a combined implementation of Lemmy and Mastodon that works with both. You can view and post to Lemmy instances via Kbin and vice versa. That said, much like Lemmy it’s still being developed, so there are still bugs and inconsistencies that sometimes cause Kbin and Lemmy/Mastodon to not quite communicate with each other properly.

The one thing I’ve noticed is that Kbin’s federation with blahaj specifically is a bit lagged. Posts only show up after a pretty significant delay, usually about an hour. Also I’ve yet to be able to successfully make a top-level image post from Kbin to any other instance, so that’s annoying.