You should use the check_cluster command for that. Here is an example :
define service {
use TMPL-S-MTX-ExternalDNS_BC
service_description Ext DNS Hosts
host_name nagios
check_command check_host_cluster!"Ext DNS Cluster"!1!2!$HOSTSTATEID:dnsux12$,$HOSTSTATEID:dnsux13$
}
You will still get alerts if one of them is down, unless you disable host notifications for them individually.
if you define a cluster, would you expect it to appear in the web interface?
I am doing a similar test to verify that at least N of 8 DNS servers are up, but I can’t see anything that makes me think nagios is aware of the cluster.