QMainWindow separator can not have different width for horizontal and vertical

I want to create a QMainWindow with two QDockWidget's.
One to the left and one in the top area.
Further both separator shall have a different width. This is not possible.

I tried the following stylesheets:
A)
-----------------------
QMainWindow::separator{
width:5px;
height:20px;
}

B)
-----------------------
QMainWindow::separator:horizontal {
background-color:blue;
width:5px;
height:5px;
}
QMainWindow::separator:vertical {
background-color:black;
width:20px;
height:20px;
}

In case A) the width of both the horizontal and vertical separator are 20 pixel.
In case B) the width of both is 20 pixel too but each separator has the associated color.
I also tried different combinations of case B) where only height or width was defined.