Jokes about canines and alignment have been done to death.
These dogs are 2 RHD magical beasts. Their chassis isn't anything out of the ordinary: good physical stats, a small wisdom boost, a charisma penalty, and a large intelligence penalty (note that death dogs are by default sapient, though). They get two 1d6 bite attacks, both of which carry a disease and allow for a free trip attempt.
Is this worth playing? At level 2, I can see death dogs being fairly competent party members. The additional head helps keep their damage output high, and your rogue and fighter will be grateful for the tripped enemies. However, as magical items start to become more important and martials get their iteratives, the death dog will rapidly become less useful.
In a low-level campaign, this is viable, and for that reason I'm inclined to go with +0. At higher levels, don't bother.