Visio 2005 allows you to open visio multiple times - does this not work in 2007? Try opening a visio document, and then starting another instance of visio from the Start-->Programs menu.
If not, read on...
Visio is an MDI interface - you'll need to stretch the whole visio window across the two monitors in question, then choose the "Window" menu and select "Tile" after you've opened your two documents.
Alternately, in the upper right hand corner just below the application minimize, restore and close buttons you'll find the document minimize, restore and close. Choose restore, and you can manipulate the windows inside the main visio app.
Hope this helps!
-Adam Davis