I think what you are effectively looking at is

$$\ \begin{align} \log(S*{AUDCAD})&=\log(S*{AUDUSD})\pm\log(S_{USDCAD})\\ \Rightarrow z&=x\pm y \end{align} $$ Thus,

$$ \sigma*z^2=\mathrm{E}\left(\left(x\pm y\right)^2\right)- [\mathrm{E}(x\pm y)]^2 =\sigma*x^2+\sigma*y^2\pm 2\sigma*{xy} $$ Hence, $$ \tag{1} \sigma*{xy}=\frac{\sigma*z^2-\sigma*x^2-\sigma*y^2}{\pm 2} $$

Does that work for you?