after some thought, I think there is no difference between multitple server or single server.
right now we are using tabbed window, if you want to monitor different server, you just need to single click the server, and the performance will be updated against the current server.
if you want multiple tab for different server, still you need to shift the tab, right?
maybe you want to see the "history" of the performance, then it make sense. is that what you need?
right now you can open multiple instances of sql monitor to achieve your goal.