Multiple headers for real_ip_header

I have a setup where there is a CDN and a loadbalancer in front of nginx. CDN will enter client's IP into the True-Client-IP header and loadbalancer into the X-Cluster-Client-IP header.

Some of the requests come through CDN and some come directly through loadbalancer, I am now wondering how to handle this situation with real_ip_header, is there a way to instruct it to check first the True-Client-IP header and if it is not existing then proceed with X-Cluster-Client-IP?

Thank you for your suggestions and help.