I tried defining the Pi-hole IP 1 and 2 as User defined DNS1 and DNS2, but haven tried what you are suggesting. Let me give it a try if that solves the problem.
Edit: I don't have the DNS director option, is that another add-on/script? I have RT-AC68U
I know you can assign a single DNS server for each client. I was asking if there is a way to specify 2 DNS servers. I have several devices assigned static IPs but they only take primary router as DNS instead of utilizing both DNS 1 and DNS2 configured servers.