I remember seeing similar question on Topscore Pro and DAT Achiever. It won't appear on the DAT but will solidify your stoichiometry concept:
At the point of neutralization, you already have 20 + 20 = 40 ml of solution.
The additional amount of HCl needed to produce a solution of pH 2 where [H+] = 0.01:
0.01 = x/(40 + x)
0.4 + 0.01x = x
x = 0.4
So Sean was correct. The answer is 20 + 0.4 = 20.4 ml