You could use check_file_ages_in_dirs on both nodes but disable the notifications and use a third/meta service with some Icinga2 DSL magic to ensure that you have exactly one check_file_ages_in_dirs on one node that is OK.
Have a look at: Calculating a state over multiple services - #4 by moreamazingnick
Or use the Icingaweb2 Business Process Module to make sure only on is OK and use the icingacli as check for the third/meta service.