By a strict reading of the rules, they do not stack. Similarily, an ex-barbarian 10/dwarven defender 10 has only DR 6/-, not DR 8/- as you might expect.
In my campaigns, however, I generally let all extraordinary (DR x/-) stack, taking precedent from certain feats in Savage Species and the Epic Level Handbook. Usually DR of this type doesn't reach a level where it breaks game balance, unlike the supernatural variants of DR, which could otherwise spiral out of control.