I think that can be done, just some css.
The li s and ul for the main menu get their own css and them for the submenu you give ul li ul & ul li ul li its own style.
However I think your solution with two seperate calls is easier, indeed because you can stick them in their own div. And its working isn t it?
You could also combine wayfinder and ditto, look
here, very unstyled, it a codaslider test, but you get the idea.