The following are the functional requirements of GTM or an equivalent feature within CDN:
GTM provides one public IP address. The system arbitrarily routes a request with the IP address as destination to either of the two (or more) public IP addresses.
You can configure the ratio of traffic between the IP addresses at GTM. For example, you can achieve a fair load balancing across different sites with a ratio of 50:50.
GTM actively monitors the IP addresses for responsiveness through a liveness object/URL. If it discovers that a particular site is unavailable, it redirects the whole traffic to the active site, irrespective of the configured ratio.
GTM IP address is always available. It is never unresponsive.