I am sorry that I am very late to this party, not least because 2014 is already out!

But in case it matters - you can open multiple models under windows, and have been able to do so as long as I can remember (at least since SketchUp 6). It's not perhaps in the obvious way (from sketchup use file -> open while another model is already open) but it works all the same - just double click a model file while another is open.