I mean, largely to me this is fine and great as long as the reverse is also true. It's fabulous to have two totally independent systems that are fully interoperable to such an extent.
I don't think there's a meaningful competition in growth or anything, that's just a number. The main downside is reduced development focus...
But -- If Lemmy is like a frontend for kbin and vice versa, isn't that fine? The Lemmy apps will load kbin posts and kbin apps will load lemmy posts.