navigation arrows – hover split in left and right

If you are referring to the jQuery mode with Nivo Slider, once it used to embed navigation arrow controls within the source script.

Now that the Combo Slider plugin is updated to the latest Nivo Slider (version 3.2), additional code has been inserted when initializing the slideshow javascript in order to achieve the same behaviour.
(see directionNavHide in the Nivo Slider support forums)

So you could edit the afterLoad lines in combo-slideshow-plugin.php and in views/default/gallery.php and detect hovering on the L/R sides as suggested here.

If you like it, you could also add a fade-in effect instead of sudden appearing of arrows (tips here), but I hope to manage to implement this in a forthcoming release