I use VS TFS extensions to subscribe this event. Alerts Explorer as far as I remember. I test those subscription using db query and TFS API, they are updating fine.
I set web service address, event type as BuildCompletionEvent2 and filter expression to pass event just for two project right now. I use OR operand to combine two project names. Both projects were selected from drop down list if that what you mean.
Update: I have interesting but rather disappointing news regarding this matter.
It appears one of the few dozen of build notification for second project make it. Too bad the service returns an exception for that.