Thanks!
So weapon category (as it is called in the Two Weapon Combo skill) is the same as weapon class? Would make sense.
Think of "Class" as a "Group of Groups", and "Category = Group".
So there are basically two different skills for dual wielding with two different weapons (from two different classes - Two Weapon Combo) and for dual wielding with two weapons of the same Kind (or of the same class?) - which would be Paired Weapons? And the latter (the one from ML) doesn't give extra attacks, but modifies the criticals on a succesful attack?
It just seems counterintuitive that you can't use Two Weapon Combo with, e.g., a short sword and a dagger, since both are in the same weapon class, and only one of the two weapons can be the primary weapon of its class.
I can certainly understand how you'd read it that way. Let me give you an anecdote. I was a member of a theatrical combat troupe called "The Ring of Steel" for roughly five years. The first live steel weapon we learned was the rapier, using cut and thrust moves. After certifying that choreography, the next weapon I learned was rapier & dagger. While both weapons were used for both attacks and parries, you did not do two attacks at the same time you parried with one of your weapons. Basically, we were switching back and forth between Shielding Weapon (Rapier & Dagger), Two Weapon Combo (Rapier & Dagger), and Short Blades (as HARP doesn't have a Combat Style to represent dual wielding while using a larger weapon as the shielding weapon).
Shielding Weapon gives you a DB bonus of up to half your rank in it (max +50), and if one of the weapons in the pair has a DB Bonus, you get to add that too. It also requires that both weapon groups have a rank of at least your rank in Shielding Weapon (x & y).
So by HARP's rules, Ring of Steel was cheating by 1. using the rapier as the parrying weapon sometimes, and 2. not having a separate dagger form (although knife fighting training and short swords may count).
HARP has both the options of refocusing your weapon and taking Multiple Weapon Proficiency.
In summary, best to think of it as trading off between two attacks (or a single attack in the case of paired weapons), and using a weapon as a shield while attacking with the other.
The Duelist Training Package in Martial Law gives equal ranks in Two Weapon Combo and Shielding Weapon, heavily implying they're intended to be the same pair.