[Maya-Python] Sub menus in shelf button

[likage]

Hello, I need some help with Mel in which I am trying to attach a sub-menu
onto a shelf button.

If the shelf button is toggled by left-mouse click (also the default), a
menu will be shown.
And if it is toggled with a right-mouse click, a different menu will be
shown.

This is my code:
shelfButton
-enableCommandRepeat 1
-enable 1
-noDefaultPopup
-width 34
-height 34
-manage 1
-visible 1
-preventOverride 0
-align "center"
-label "Set Rig Options"
-labelOffset 0
-font "tinyBoldLabelFont"
-image "myTestIcon.png"
-backgroundColor 0.25 0.25 0.25
-style "iconOnly"
-marginWidth 1
-marginHeight 1
-sourceType "python"
-command "import moduleA; menuA=moduleA.dropdown_from_selection();
moduleA.showMenu(menuA)";

// When right-mouse click is toggled.
popupMenu -b 3;
// I need to give the label a name, otherwise Maya will
auto-provide a name such as 'menuitem1993' etc.
menuItem -l "MenuB Label" -sourceType "python" -command "import
moduleB; menuB= moduleB.dropdown_from_selection();
moduleB.show_menu(menuB)";

FYI - both menus - menuA and menuB are created/ generated using QMenu.

However, in the case of right-mouse click, it has to go through the label,
before the actual menu is shown. Is there anyway, in which I can bypass
this label, or if there is a better way in which I can handle both left and
right mouse click?