Run two and check the logs. You’ll see about 20% of your requests will log on the second instance. So currently, that’s 20% of your DNS requests not being filtered.
You’ll also find some devices just latch on the the second and never use the first - again, in your scenario, these are not being filtered.
That’s not how the two entries for DNS works. Devices will use both rather randomly, and therefore some requests will not be filtered.
The best way is to run two instances for redundancy.
You can take the community and server name and format it like this: https://lemmy.world/c/[email protected]
But kbin is more than just posts - it also federates Mastadon which I don't think Lemmy can process (so you won't see those).
(I'm still working all this out myself -- in fact, this is my first ever comment!)