Priority group activation on GTM.

If you're referring to the WIDE IP pools that contain the virtual servers discovered from server objects, you can use "Global Availability." This load-balancing method allows the F5 to always resolve the request from the first entry (the active one) and only respond with the second entry if the first one fails.

If this is not the case, and you're talking about the "Delivery > Load Balancing > Pools" configuration to load balance traffic across two DNS servers, you can follow the link shared by Jmtaylor. The pool configuration is similar to the LTM pool setup. In this scenario, you can use the basic Priority Group Activation with a condition of "less than one" if you have only two members.