Ok, you have caught me out. I do develop on a Mac computer. But my client uses windows and I have observed them stretching a single browser window (ie IE or Chrome) over dual monitors. You are still restricted to the underlying browser window when you drag your modeless windows, but when that browser window is stretched across multiple monitors then the modeless windows can also be displayed on the different monitors.
I am not sure if this requires special settings on Windows. I am not in front of their computers for a few weeks but I can check their particular setup then if you need.
Simple diagram of what I mean :


