  Let $(\alpha,\mathcal{N}_{\alpha})$ and $(\beta,\mathcal{N}_{\beta})$ be two
canonical number systems for an imaginary quadratic number field $K$ such that
$\alpha$ and $\beta$ are multiplicatively independent. We provide an effective
lower bound for the sum of the number of non-zero digits in the $\alpha$-adic
and $\beta$-adic expansions of an algebraic integer $\gamma\in\mathcal{O}_K$
which is an increasing function of $|\gamma|$. This is an analogue of an
earlier result due to Stewart on integer representations.
