does federate with each other mean you have one account and can log in into lemmy and kbin with that one account? or is it you can use lemmy with a kbin account and vice versa?
You log in to your home instance (kbin or Lemmy, doesn't matter which) and you can follow and participate in communities hosted on other servers, which can be kbin or Lemmy, doesn't matter which, because they support a common protocol (ActivityPub).
In fact, you can even interact with communities hosted on a Lemmy instance from a Mastodon instance, or Misskey. Heck, probably Pixelfed, though I haven't checked. Such is the beauty of the fediverse and protocols over platforms.
I think we need to start referring to Lemmy, Kbin, and presumably the dozen or so other 'ActivityPub with a Reddit-like interface' projects which will inevitably pop up collectively because while they compete the dynamics of that competition are in theory quite different to what you'd expect with centralised platforms as I can interact with communities on Kbin just as well from my 'home' Lemmy instance.
I quite like the term 'Threadiverse' that's been going around lately.