I believe this only works if your ad blocking DNS is configured to return 0.0.0.0 for all blocked domains rather than NXDOMAIN, since then services might try using the secondary DNS instead and that would result in nothing getting blocked. Ideally your secondary DNS should be a copy of the primary.
do you know if pihole or Adguard can configured to support confirming to the router or the client that resolution took place, rather than try the secondary DNS.
If i understand you correctly, if you have a blocking internal DNS running pihole or Adguard and an external general DNS such as google or cloudflare, unless what you described can be configured, the requests that come back "blocked" from pihole would then simply be resolved by google/cloudflare, thus negating the point of pihole.
AdGuard Home should by default be configured to return 0.0.0.0, you can check whether that's the case in Settings -> DNS Settings -> scroll down to Blocking Mode. I don't know about Pi Hole but it probably also has a similar setting.
