Imo it does not stack
In the recapitulative table page 72 of the PHBII the Benefit from Melee Weapon Mastery is stated exactly in those terms:
"Gain Weapon Focus, Weapon Specialization benefits with multple weapons"
In the full descriptor page 81 you can read:
".... Your training with one specific weapon now EXTENDS to other weapons of a similar sort..."
Quite clearly, considering both the short description stating it's a Weapon Focus and Weapon Spec., and the description stating it is an extension, and not an addition, there is no doubt, the feat does not give you an additional bonus to attack and damage, it justs lets you extend WF and WS to a wider category of weapons.
The discussion stem from another passage in the descriptor that says "...when using any melee weapon that has that damage type you selected, you gain a +2 bonus on attack rolls and a +2 bonus on damage rolls."
The "+2 bonus on attack rolls" is obviously a typo, and should read "+1 bonus..." as per Weapon Focus skill.
This is not to say that a skill that would add +2 attack and damage rolls over WF and WS would be a bad thing, just in this instance, there is no doubt to me as to what the skill does.