You would create tags for the MAC of each device, assigning the IP which corresponds to the correct instance. There's also examples in the forum you can search for. A common use case is for parents to assign different kid-friendly DNS servers to specific devices. In your case, you describe each IP being a different DNS proxy.
You are the person making the configurations. Also, I've never heard of someone doing this.
It's definitely possible, I'm just not sure if it will accomplish what you desire.
That's already understood. I think you misunderstood my concern. Those IPs are the same subnet, but you were discussing "relaying". This implies you may have a misconfiguration.
But you assure us that the device is properly configured.