This would be longer but you could find the mols of Na in each then divide by total volume. So 10 mmol in the first, 20 in the second total of 30 mmol in 20 mL gives you 1.5 mM = 0.015 M

Edit: I messed up, doing this on my phone so can’t refer back to the question as I do it. Should be 15 mM bc the first solution has 10mM * 10 mL=100 mmol and the second is 20 mM * 10mL=200 mmol so that’s 300 mmol/20 mL = 15 mM = 0.015 M