What monitors the monitor-monitor?
If you are wondering how to monitor if both data centers go don't at the same time, I'd say for most companies you don't worry about it. 1) The odds are extremely low. 2) You will notice if two DCs go down. 3) That nightly email that says "I'm up" isn't going to help here. 4) Even the free version of Pingdom will alert you when your whole datacenter is down.
There are all sorts of other things to consider with redundant monitoring, but that's the job of a sysadmin -- identifying failure points, assessing risk, etc.
Usually folks divide the monitoring work among two servers and each server monitors the other. Or, you "meta monitor"... a monitoring system that just monitors the monitoring system. Then you get a third-party to monitor that. Then it is turtles all the way down.
For a real DC, when it goes down I get a phone call from a human. I don't have to reinvent that process. If it's my own server room, I use a landline and a modem for OOB "dude you gotta come down here" notifications, a WAV of Woody Woodpecker or something. If the phone lines are down, I look at the newspaper headlines to see what happened.
There's no reason not to set up a standalone monitoring regime. Whether or not you use heartbeat notifications to tell you all is well is a matter of taste, but there is definitely more to maintaining your nines on a daily basis than simply adding more layers of monitoring.