You can use either or both of these as upstream DNS servers, but they will be in parallel, not series. Traffic will selectively route to the upstream server that is the best performing (see link below). If the traffic from Pi-Hole goes to Cloudflared, it won't be going to the OpenDNS service.
I looked at the OpenDNS site and it does not appear they support incoming DoH or DoT traffic, so it does not appear they can be configured as an upstream server with Cloudflared.
If your goal is filtering inappropriate content, then I would drop Cloudflared and route all the traffic to the filtered DNS provider. If your goal is to hide your DNS traffic from your ISP (or whatever reason you use encrypted DNS), then you would drop the filtered DNS and send all the DNS traffic through Cloudflared.