You can use 1 monitor for HTTP/1.1 if the host header used is the same between the two hosts. It doesn't matter if a VS is created yet or not. That's independent. There's no connection between the VS and the monitor except that the status of the VS inherits the status of the default pool which is a result of the monitors results.
So if both your pool members respond with a host: header of mybestwebsiteever.com use that... or use what they think it will be when it goes live... As long as the host likes it, you're golden.
H