Define $K_n'$ to be the graph obtained from the complete graph by subdividing each edge once. Let $G$ be a graph with $\chi(G)=c$ and $\eta(G)=h$. Define $2G$ to be the disjoint union of $K_{2h}'$ and $K_{2c}$. Assuming Hadwiger's conjecture, we have $c \leq h$, and so $\eta(2G)= 2 \eta(G)$ and $\chi(2G)=2\chi(G)$ (since $K_n'$ is 2-colourable for all $n$).
Of course, this may not be the type of construction you had in mind, but it works (assuming Hadwiger's conjecture is true).