Nothing in the rules state that double weapons behave differently when targeted by sunder, so I’d expect that a single successful sunder would break the entire weapon. It does not seem possible to fight on with the other end after one has been broken. This inclines me to feel that a weapon, as a singular object, can only have one HP and hardness value.1 Thus, though the two ends see independent enhancement bonuses for determining their attack and damage bonuses, the weapon as a whole sees two overlapping enhancement bonuses when determining the HP and hardness.
Since enhancement bonuses do not stack, the weapon uses the higher enhancement bonus to determine its HP and hardness.
The bigger confusion is actually weapon materials. The rules say that you can create the two ends from different materials, but then never explain how that works for things—like HP and hardness, and also weight—that belong to the weapon as a whole, rather than to a single head. Again, I would probably just treat them as overlapping, but that is a bigger stretch than with enhancement bonuses.
Ultimately, double weapons are a mess; these are not the only confusions that arise from their use. And due to the need for Exotic Weapon Proficiency (aside from the quarterstaff), they tend to be rather poor weapons, too. And they are massively unrealistic (quarterstaff again excepted). I happen to think that they’re pretty cool, myself, and don’t care too much for realism, but ultimately they just have so many problems that I just avoid them.
- I am not aware of any rule that comes out and says this, but any other interpretation leads to either having to make pretty big assumptions about sundering double weapons, or just plain nonsense. So I’m invoking Occam’s razor here: singular HP and hardness values requires the least speculation and fewest assumptions on our part.