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.

DietPi is the perfect OS with easy install software for Pis of all flavours.

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.