a "damage bonus" is not a type of bonus, as you can tell by looking at the table in the DMG/SRD (I don't have a page number...) that says the different types of bonuses. Also, epic feats are generally slightly more powerful than regular feats, so it stands to reason that epic focus would add a +2. Also, since it requires weapon focus as a prereq, you would think that they would think to tell us if it didn't stack (as they do with all other occurences of feats like this, such as greater spell focus). Finally, the bonus type for both weapon focus and epic weapon focus are unnamed, and thus are not subject to the stacking rules (that is, they always stack...they are not from the same source, either, they are from different sources (epic weapon focus, and weapon focus)).