template <typename Type>
class Outer
    {
        public:
            template <typename InType>
            class Inner
            {
                public:
                    template <typename X>
                    void nested();
            };
    };




надо написать
template <typename T1, typename T2>
class Usage
    {
        public:
            void caller(Outer<T1>::Inner<T2> &obj)
{ вызов obj. nested<int>(); }
    };