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,

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

Does that work for you?