I have heard many times, consistantly, that you should never pair 2 puffers together.
However, at my lfs - they have 2 dog face puffers together and there is no agression. I don't know if that will change later when they get bigger. I think size of the tank and potential size of the puffer species has a part to play in the chances.
I love puffers, they are my favorite, but I am very reluctant to put more than 1 puffer together.