(1) $\lim \limits _{ {x \rightarrow 0} \quad {y \rightarrow 2}} \dfrac { \sqrt {1+xy}- \sqrt {1-xy}}{x}= \underline { \quad \quad \quad }$.
(2) $\lim \limits _{ {x \rightarrow 0} \quad {y \rightarrow 2}}( \cos xy^{2})^{ \dfrac {1}{ \ln ^{2}(1+x)}}= \underline { \quad \quad \quad }$.