Unable to create two topmost windows

#include <QApplication>
#include <QWindow>
#include <QTimer>
#include <QDebug>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QWindow b;
    QWindow c;
    b.setFlag(Qt::WindowStaysOnTopHint);
    c.setFlag(Qt::WindowStaysOnTopHint);
    b.setPosition(1200,700);
    c.setPosition(1280,720);
    c.show(); // <- is not a topmost window
    b.show(); // <- is a topmost window

    return a.exec();
}