Did you checked AGH wiki? I guess option is wrongly implemented
https://github.com/AdguardTeam/AdGuardHome/wiki/DHCP
IP’s needs to be comma separated, isn’t it? And one IP should be enough. As well your clients would need to reconnect to your network to get new DNS server information. These information are assigned only if the client requests an IP address.
Yes, I tried that and couldn’t get any access with the Rpi off OR on.
The syntax is a bit ambiuous because the default file is “options: []” so I initially included the square brackets. That produced the syntax I posted above. I then removed the brackets and entered the DNS as shown in the wiki for 2 IPs: ‘options: 6 ips 9.9.9.9,149.112.112.112’ without the quote marks. That prevented access when the Rpi was off OR on.
But I will try again with just one IP.
if thinks are not working as expected, just ask for help on your open AGH issue. I guess these guys are more experienced than we are.
It appears that what we were hoping for wouldn’t work as planned, so I’ll live with no redundancy 
Per github:
However, what you are trying to do won’t work as you expect it to. The DHCP server will return 2 DNS servers to the clients and the clients are free to use whichever one they want and switch between them. Even though they are called primary and secondary, there’s no order implied, they are considered equivalent. So some DNS requests will go through your AGH, others will go directly to Quad9 and won’t be filtered.
If you want redundancy, you need 2 installations of AGH, on 2 devices, configured the same way. For now you’d have to maintain the configs in sync manually, though see this issue #573
well my PiHole+Unbound is working 24/7 since ages and did not shut down. Personally I don’t think redundancy would be needed. 
If there is a real need, just go for a 2nd device.
Yeah, my setup been very reliable too.
Thx again for your assistance, Joulinar.