valar007
Discoverer
I would say that would be a pain to (setup and) maintain. Both the DNS on the router will point to the 2 PiHole instances. It's like having 2 main doors for a house.Yes, different physical hosts.
My PiHole installation on a RPi 3B+ had an uptime of over 600 days until I decided to upgrade. For the majority of the time, my config was PiHole + unbound + Quad9 DNS. On the router, only one DNS was configured, the PiHole's. But one fine day, internet was down and after debugging for 1 hour, got to know that Quad9 was down. From that day onwards, on my router, the 2nd DNS points to CloudFlare. This way, I have 3 routes.