The two Kurama did combine at the end of Part II, after Yang Kurama was re-sealed into Naruto. Naruto always had access to the full Nine-Tails' form, his Kurama Mode and Tailed Beast Mode are just superior. Naruto used the Nine-Tails' full form in the Last. Naruto proved to have the form accessible, because, prior to gaining his Torii Seal and Nine-Tails' Chakra Mode, Kurama would attempt to achieve its full form as its tails increased. During the battle with Pain, it had advanced almost entirely to its full form, it was just missing its ninth tail, skin, and fur.