I have several Lemmy and a few other fediverse accounts, use what you want however works best for you.

What is bin or what is a bin account?

Several account as well. Is not a dogma, just do what you want.

I had a kbin account from the initial exodus, but have become exclusive to my current instance, because it’s functional with Voyager and the instance is relevant to my interests.

E: Fixed some meanings.

You just used kbin account to ask a question in lemmy community, I think that answers your question.

I am also following few kbin communities from my lemm.ee account

There are two main differences at this point: MBin (KBin itself is dying as the dev had some combination of burnout and undisclosed personal shit) has better Mastodon integration (if slightly confusing), but worse app support. For “Reddit like” functionality, they’re pretty much completely comparable and compatible. If you’re happy with your setup, no need for two accounts.

I created also mbin account (on fedia) to try it. I used it mostly as a backup when SJW it down however, over the year, it really improved. My lemmy account is still my main, but at a point in future it may change

You can use both. There is likely to be a lot of overlap, but no matter. You just have to remember when to say magazine and when to say community I guess?