已知正项等差数列\(\{a _{n} \}\)与等比数列\(\{b _{n} \}\)满足\(a _{1} =1\)，\(b _{2} =4\)，且\(a _{2}\)既是\(a _{1} +b _{1}\)和\(b _{3} -a _{3}\)的等差中项，又是其等比中项．
\((\)Ⅰ\()\)求数列\(\{a _{n} \}\)和\(\{b _{n} \}\)的通项公式；
\((\)Ⅱ\()\)记\(c _{n} = \begin{cases} { \dfrac {1}{a_{n}a_{n+2}},n=2k+1} \\ {a_{n}\cdot b_{n},n=2k}\end{cases}\)，其中\(k∈N*\)，求数列\(\{c _{n} \}\)的前\(2n\)项和\(S _{2n}\)．
