iRule to update vip address that DNS entry is resolving to.

I may be misunderstanding what you are trying to do, but it sounds like you could just have GTM do this without an iRule.

You'd create your wide IP for whatever the FQDN of the service is, and have your pool built with both cluster IP addresses as pool members. You'd setup a monitor on the pool to check the status of the clusters and modify the answer handed to the client based on the health of the pool members.

You can select different types of load balancing methods to do what you want. Least connections, ratio, round robin... Tons of options. Pick one that will best suit your environment.