Law of multiple proportions is applicable when two or more elements combine in more than one form.
$\mathrm{NaNO}_3$ (Sodium nitrate) and $\mathrm{CaCO}_3$ (Calcium carbonate) are completely different compounds involving different elements (sodium and calcium with nitrogen, oxygen, and carbon). These compounds do not combine in simple whole number ratios between their constituent elements that would illustrate the law of multiple proportions. So, this pair does not demonstrate the law of multiple proportions.