Metal carbonate + carbonic acid?
$\ce{Na2CO3 + H2CO3 -&gt; 2NaHCO3}$, yielding sodium "bicarbonate", as an example, and in similar fashion, $\ce{Na2SO4 + H2SO4 -&gt; 2NaHSO4}$, or sodium "bisulfate". The "bi" in this nomenclature meant two different anions, and is confusing, as it is the opposite of "bi" in mercury bichloride, two of the same cations. For that reason, sodium hydrogen sulfate or sodium acid sulfate might be preferred.