Nagios Support Forum

I wouldn't try setting this up with service dependencies. The easiest way to do this would be via the Nagios BPI.
Create a new BPI Group, add hosts A & B (these would be the ping checks for both hosts), set the "Health Thresholds" as "Warning" = 49 and "Critical" = 48.
If one of the two hosts goes down, the status of the BPI group would be still "OK". If both go down, status will change to "Critical".
After you are done, run the "BPI Wizard" to set up check for your BPI group.

thank you very much for you guidance but actually what i want to know is that notification(email ) to be sent when both of them are down is it possible ?( currently i configured bpi group but it send email when one of then is down.)

It should be yes, you most likely will want to look at using the bpi wizard within nagios to check against the status of your groups. This way you can get alerts specifically when a group is in a critical or failed state. If you are looking to modify the alerts already coming from nagios for a service that is dependent on two other services, you may wish to disable or alter notifications for that dependent service and use the bpi check for most notifications.