Metal phosphate buffers
I think phosphate buffer is the solution here. Reason:

$K_\mathrm{sp}$ of $\ce{CrPO4}$ at $\pu{25 ^\circ C}$ is $2.4 \times 10^{-23}$ while that of $\ce{Cu3(PO4)2}$ at $\pu{25 ^\circ C}$ is $1.4 \times 10^{-37}$, thus both ions will precipitate since concentration of each ion is $\pu{0.02 M}$.