is there result relating $(a b)^{x} \text { with } \bmod (m n)$? 
$\begin{array}{l}{\text { if } \operatorname{gcd}(a, n)=1 \quad \& \operatorname{gcd}(b, m)=1} \\ {a^{x} \equiv s \bmod (n)} \\ {b^{x} \equiv s \bmod (m)} \\ {\text { is there result relating }(a b)^{x} \text { with } \bmod (m n) ? ?}\end{array}$


Note that
$$a^{x} \equiv s \pmod n \iff a^x - s = jn \; \text{, for some} \; j \in \mathbb{Z}  \tag{1}\label{eq1}$$
$$b^{x} \equiv s \pmod m \iff b^x - s = km \; \text{, for some} \; k \in \mathbb{Z}  \tag{2}\label{eq2}$$
Multiplying \eqref{eq1} with \eqref{eq2}, and switching sides, gives
\begin{align}
(jk)(mn) & = \left(a^x - s\right)\left(b^x - s\right) \\
& = a^x b^x - a^x s - sb^x + s^2 \\
& = \left(ab\right)^x - s\left(a^x + b^x - s\right) \tag{3}\label{eq3}
\end{align}
Thus,
$$\left(ab\right)^x - s\left(a^x + b^x - s\right) \equiv 0 \pmod{mn} \iff \left(ab\right)^x \equiv s\left(a^x + b^x - s\right) \pmod{mn} \tag{4}\label{eq4}$$
As discussed in notation - What is the difference between $x \bmod y$ and $x \pmod y$?, if you're indicating a binary operation with the smallest positive remainder, i.e., so $0 \le s \lt n$ and $0 \le s \lt m$, instead of a congruence relation, and you want to determine the smallest such remainder for $\left(ab\right)^x$ when divided by $mn$, then it'll be the right side of \eqref{eq4} possibly adjusted (most likely reduced) by some multiple of $mn$. However, without having more information about what the various values are, I don't see any way to provide a more explicit answer.
