They are two separate projects with similar goals (implement a Reddit clone). They both use the activitypub protocol, so they can generally interoperate. Other activitypub based services, such as mastodon, can also interact with either Lemmy or kbin, but in a more limited and clunkier way.
Lemmy was started first so there are more servers and more users, kbin is more recent.
Kbin saw more relative growth in comparison recently though. How well they can communicate with each other always depends on the implementation. For example, I think the microblogging in kbin could definitely be more fleshed out to be able to communicate better with actual Mastodon. Right now it goes more in just one direction.
We can see the posts on both platforms (except for the case of de-federation).
Competition is an odd topic for open source software. For example, whilst mastodon, kbin and lemmy are all competing for attention, they are collectively making the activitypub ecosystem more attractive.
A similar thing happens with linux: Oracle, red hat, canonical etc all compete for market share, but pay for developers to work on the wider linux ecosystem (the kernel as well as GNOME and other apps etc)