As shown in the article “Setting up Qt framework with VS2008 for WinCE with Toradex modules” I must select an SDK either “Qt 4.8.4 OpenGL SDK” (WEC7) or " Qt 4.8.4 SDK" (WinCE6). But is it possible to install Qt support for both v6 and v7, so that I will be able to build the same project for both platforms?
Yes, you can try to install both SDK in a machine and develop your application. Theoretically, we are not seeing the any blocking factor, you might be required to do some rename while installing the QT. But we never tried and it is not recommended.
There is no support for QT 4.8.4 from QT, also we don’t have in-house expertise. QT 5.x is released and that is not supported for Windows Embedded Compact, it is supported for Embedded Linux.