Control widget positioning with multiple desktops with multiple screens

Hello,

I'm developing an application in an environment with three screens with two GPU's.
We have two separate desktops, the first desktop with two screens ( screen1 and screen2, atached to a single GPU) as a virtual desktop, and the second desktop with one single screen ( screen3 atached to the second GPU) .

Code:

---

screen1            screen2                 screen3 
.-------------    -------------        -------------
|             |  |             |      |             |         
|             |  |             |      |             |
|             |  |             |      |             |
.-------------    -------------        -------------
           desktop1                       desktop2

---

I see QDesktopWidget gives only information about the screens and I need to have some control to put my widgets on the screen I want. For example, I can't find how to change the primary screen of the application ( at runtime, obviously ). Moving is not the solution because they are separate desktops so I can't move a widget from screen2 to screen3 for example. Another useful feature for me would be to attach the main window to a default desktop, independently from the desktop I'm invoking the application.

I'm working on linux as the main platform, qt 4.2.2, but I would like to have the most portable solution to control where I put the widgets in the available screens