I finally got it - you're talking about pure Start screen apps such as Maps which open to full screen only (hiding the task bar as well of course) so you can't have more than one open yes? So don't use Metro Apps. However for all ordinary programmes, from the Start screen I can open as many Firefoxs or Chromes as I like - they open on the desktop of course because the desktop is still the core of Win 8.
I think we all should make it clear when talking about Apps if we mean desktop shortcuts we place on the Start screen or Metro Apps. They are different and behave differently. It also seems odd to refer to Metro as an interface. On a desktop PC Metro is just a bolt-on that sits over the desktop. The desktop is always there. Also, once opened, a Metro app is always open, there is no mechanism for the user to close it even when not visible.
Metro seems to be of little use to desktop PC users beyond that I've found its Start screen useful for organising all my desktop shortcuts in handy groups. Otherwise Metro rarely impinges on me. That's my latest take anyway but I'm still learning and exploring!