Standard selling tool, no option to select second category:

Advanced selling tool, option to select second category:

OP, as I said in my last post, go into change category, then click 'switch to advanced tool' up the very top right of that screen. It probably means you have to start the listing again, as opposed to just revising it, but at least you will have the option of 2 categories. 

I took my screen shots by hitting revise in one of my current listings and when I switched to the advanced tool, it took me to the start screen where it asks you to to put in a title, hence my assumption you have to start the listing from scratch.