This is impossible because, as you may have some humidity and even carbon dioxide in the atmosphere, the sodium oxide is likely to be converted to sodium hydroxide, sodium bicarbonate or sodium carbonate or even to all three compounds. So unless you can define the specific atmospheric conditions, it is not possible to guess how to determine the concentration of both. However, if you know that carbon dioxide and moisture are not present, you can determine NaOH and Na2O as total NaOH concentration by dissolving in carbon dioxide-free water before titrating against known acid concentration. If you subtract the NaOH concentration determined by your already established method from the total NaOH concentration, it will give you an estimate of the Na2O concentration.

This is with respect to Dr. Adeloju's answer. It is difficult to determine NaOH and Na2O from their mixture but it is not impossible. In the context of source term study of sodium fire aerosol some investigators have reported the presence of Na2O , NaOH and Na2CO3. But the technique for determination was not mentioned. I feel by selection of appropriate organic solvent Na2O may be separated from the composite matrix. But I am not sure.

Na2O cannot exist in aqueous environments. It react immediately with atmospheric moisture producing sodium hydroxide, NaOH. Na2O can only exist in certain defined conditions - absence of water and absence of CO2.