I run pihole on a proxmox cluster (lxc containers), 2 separate IPs and I setup keepalived and made the virtual IP the primary dns ip that my dhcp server hands out, pihole1 is the master and pihole2 secondary. I use gravity sync to keep both piholes in sync. Works very well and I can reboot one at a time without losing dns at all. Techno tim on YouTube has a guide on how to setup keepalived on 2 pihole servers that helped me set it up.