setting partial options using the property setter?

setting partial options using the property setter?

I am trying to set a single sub-option on a plugin instance without overwriting the entire option.  I am working with the button widget where the secondary icon was set on instantiation of the button, and afterwards I want to set the primary icon via the property setter.  how can I do this?
 
I tried this and I end up with only a primary icon:
$("#toolbar").menubar({...}); //this creates a button widget for each button and sets the secondary icon during instantiation of each button having a flyout menu

$("#city-menu-button").button( "option", "icons", {primary:'ui-icon-gear'} );
 
Thanks,
 
Michael Lang