I wonder if you have same issue as i experienced with Asus.
If using the router for DHCP, can you post results from below on a Linux client please:
cat /etc/resolv.conf
Or if on a Windows client:
ipconfig /all
*For latter, I am only interested in the DNS server(s) displayed.
I dont have details, you'll need to figure out yourself as probably no one tried before.
Its Linux so almost everything you can think of is possible if using right software and bit of scripting.
One solution I can think of is installing cluster software with a virtual IP address (the single interface) that can failover from one Pi/system/node to the other.
This does mean that only one Pi will be active in the cluster.
For proper stats, this also means you need some kind of cluster aware shared storage, like for example a NAS, for storing logs and the black/white-lists.
And you'll need to cluster at least the dnsmasq daemon and cron jobs running for Pi-hole.
And if want it to be truly redundant, the shared storage needs to be mirrored or distributed as well so you need two NAS units or similar storage.
You asked complicated 
If you create a file in the "/etc/dnsmasq.d/" folder, like for example "20-records.conf",
you can add your personal DNS records to this file like for example (from the man page):
cname=<cname>,[<cname>,]<target>[,<TTL>]
Or for text:
txt-record=<name>,<text>
Check the dnsmasq man page for other types like for example mail:
mx-host=<mx name>[[,<hostname>],<preference>]
Ps. I believe when you add records, dnsmasq needs to reload eg:
sudo service dnsmasq reload
sudo service dnsmasq restart