While you're on kbin, the servers share basically everything under the hood, so the distinction isn't that strong. Compared to Lemmy, you'd see a different interface and some features may differ, but the underlying content is basically the same whether you're on kbin or Lemmy. So despite being different products, it's basically one big community. The likes of Mastodon are technically in there too, but the threading structure of kbin and Lemmy means you'll mostly see those two products sharing content.
Though note that the specific site you're on does control things like the sorting of posts. What's hot for a kbin user might not be for a Lemmy user. Similarly, sites could hide or block some kinds of content if they want to. Eg, I think kbin probably did something to filter porn off the front page, cause I don't see porn anymore unless I look for it. Beehaw is another good example here. It's a Lemmy instance that decided to block the biggest other Lemmy servers to limit access to their communities.
TL;DR: same content, different interface for viewing and interacting with the content.