1. Create a new project using Creator's wizards.
   I used a Qt Widgets Application project. It doesn't matter whether it's based on qmake or CMake.
2. Go to Projects mode, Build settings.
   The project has a build configuration named "Debug". Its build directory is "<source directory>/build/Desktop-Debug".
3. Add a new build configuration of type "Debug". Give it the same name: "Debug".
   The new build configuration is automatically renamed to "Debug2" to differentiate it from the existing one. Still, it uses exactly the same build directory.

The updated name of the build configuration should also be used for the build directory.