I tried to created a multi window application like this:

Item {
    Window {
        ...
    } 
    
    Window {
        ...
    }
}

 It compiles and starts but there is no window showing.

The same code is working is started via console:

<path to qt>\5.12.5\msvc2017_64\bin\qmlscene.exe main.qml

Though it opens one window more than specified with the title "qmlscene"

OBSERVATION:

• multi window app starting but not showing using the creator
• multi window app starting and showing using "qmlscene.exe"

EXPECTATION:

• some error message in the creator...
• ...or windows showing when starting from creator

Test project attached.