I'm extremely in favor of being able to interact with both Lemmy people and Mastodon people, and I wanted to switch to kbin because it seems like one of a very few solutions that can do that.

Initially I was pretty skeptical of mbin, because as a general rule the "screw the existing maintainers let's fork" people are usually mistaken about some things, but I have to say having installed and worked with both, I like mbin significantly better. My experience is minimal, and the fork was pretty recent, but mbin fixes some specific things that I had trouble with or that irked me during the short time I was working with kbin. Some significant instances also seem like they've switched over to mbin. Having kbin's "flagship" instance down for like a week didn't really help either. As I understand it, the mbin philosophy is "let's fix up the backend and get federation more solid before we do much more in terms of big new features" which I can get behind.

They're both very rough and early pieces of software, honestly as is the entire Lemmy side of things it seems to me. If you're interested because you don't want things like the 0.19 federation breakage, then I am sad to report that you might find broken stuff in kbin / mbin as well.

I honestly have no idea about the drama side of things. I wish all good things for Ernest and I'm happy with the software he 99% created. kbin has some things (e.g. combined Lemmy+Mastodon posts all in one home screen) which mbin doesn't have, which makes it kind of a shame that they're being developed apparently irrevocably separately at this point. IDK. Like I say I have no idea about that side of things.

Just my 2c as a person that installed mbin recently and likes it

(Edit: To answer the specific question, it seems like they're completely separate at this point. They split in early October with no synchronization in the separate paths of development since then, it looks like to me.)