Last week at TechEd I was showing off Query Notifications. When I showed using the preprovisioned queue (overloads on SqlDependency.Start and SqlDependency constructor), a delegate asked about using the same queue with more than one subscriber. He repeated the question when I showed the low-level SqlNotificationRequest.
I did some tests over the weekend and the answer is no. You need a separate queue for each subscriber, otherwise the SqlDependency listener gets "confused". If you use the dynamically created queues like most people do (SqlDependency.Start with only a connection string), you get a new queue and service for each subscriber. So you won't notice this. I thought I'd have better luck with the SqlNotificationRequest because I'm reading the queue myself, but without going to the system tables (and perhaps introducing race conditions) its not possible there either.
Queue are cheap to create, so there is really no reason not to create a separate pre-provisioned queue and service for each notification subscriber. But, at least for the time being, its a requirement.