I have a MBPr first generation since July 2012, after days and days researching and asking on Windows and Apple support forums repeatedly and after doing numerous hacks I have concluded that at this moment it isn't possible.
Apple was the first company to develop their OS thinking on pixel independent GUI, and the first to have a HiDPI display...
Microsoft is behind Apple on that matter, Windows 8 have some GUI elements and some programs that are pixel independent, but some other can't detect the pixel density, Photoshop is one of them. Chrome seems to detect the display pixel density but the scaling is just awful.
Windows 8.1 with the latest updates and patches fixed some GUI elements and overall achieve better results using HiDPI displays, but, there is no way to have two different displays with different pixel density.
Maybe in time, when HiDPI displays become mainstream they will fix Windows, but first, they have to fix a lot of thing on Windows, they are already on that path with the new CEO announcing the return of the start menu.