You can run multiple versions of VS (2012...I haven't checked others). If you pin it to the start menu and right-click and open a new instance it will open a new instance. Agree that this is annoying. It seems like the metro start/find screen takes the focus back to an existing instance by default, rather than launching a new instance. For a user with a fairly simple mental model of how things work this is probably a good thing, but for more advanced users it is a problem.
From some of the other comments it looks like shift+click on the metro start screen works to open a new instance (which is the same as the task bar in Win7,8)
