There might be something here:
On your DHCP server, you could put the address of pihole 1 as DNS 1 and pihole 2 as DNS 2. The way DNS works on LAN, DNS 1 and DNS 2 are both active, your clients will use both of them at random in parallel, it's not the case that they use 1 and only 2 if 1 fails. So that would give you an active/active HA setup.