Let us say that elongation in copper $=x$

Than elongation in steel $=2-x$

We know

$\frac{F L}{A Y}=\Delta x$

$\because F, A, L$ are same only material is different We can say

$\frac{1}{Y} \propto \Delta x$

$\frac{Y_2}{Y_1}=\frac{\Delta x_1}{\Delta x_2}$   $\left\{\begin{array}{l}\text { Where } \\ Y_2=Y_{\text {steel }} \\ Y_1=Y_{\text {copper }} \\ \Delta x_1=\text { elongation in copper }=x \\ \Delta x_2=2-x\end{array}\right.$

Substituting values

$\frac{20 \times 10^{11}}{12 \times 10^{11}}=\frac{x}{2-x}$

$x=1.25 \,cm$

So $\Delta x_{\text {copper }}=1.25 \,cm , \Delta x_{\text {sleel }}=0.75 \,cm$