gtm question

Hi,

 

I have two gtm that located different sites.I know that I need to create ns record for each gtm for delegation but I would like to give priorty one of them and if high priorty gtm is fail then I would like to handed traffic another gtm. As I know I need to arrange diffirent ttl time for each ns record on dns for my goal or I need to configure some setting on gtm devices? Please someone say me which one is correct?

 

Thanks,