dns cache resolver

It sounds like you'll want to have both of your GTMs in a sync group. That way, their configurations will be the same and no matter which GTM the query arrives at, both will provide the same resolution.

The authoritative DNS server that will delegate the domains to your two GTMs will need to be configured to delegate to both. When a client requests an NS record for the domain, your authoritative server will hand back both of your GTM hostnames/IP addresses. The client will then try to query one. If non-responsive, it should try querying the other. If both GTMs are non-responsive, then the query will fail.

You can configure global availability as your load balancing method on the GTMs. Keep in mind what that will do is just control the method which GTM responds to client queries. It won't affect how your parent authority delegates domains to your GTMs. If round robin will work, then global availability will too. Just a different balancing algorithm.