Secondary Damage is definitely your off-hand weapon's damage range. There are a few things to note about how Dual Weilding works in Pillars of Eternity. Should be noted that there are a lot of different stats, talents, etc that can effect some of the more specific numbers used as examples below.
- Only active abilities that have the "Full Attack" attribute will utilize both weapons (e.g. Blinding Strike for Rogues says, "Full Attack + 1.25x Damage") Otherwise the ability will state "Primary Attack", which is self-explanitory.
- Dual wielding is a rotating attack system, where each weapon takes turns hitting and recovering. The recovery depends on a number of things, the speed of the weapon, your armor penalty to Recovery (heavier armor, slower recovery between attacks). So for instance a 1handed sword is an Average speed weapon. So it takes 1 second to attack, 1 second to recovery period, and then an additional recovery period of 1 second (apx). With dual weilding, you get to replace the second recovery period with the second weapon's attack.
Example of 1-Hand Weapon fighting. (Average speed weapon)
1s Attack (animation), 1s Recovery, 1s Recovery (3 second cycle)
Example of Dual-Weilding weapon fighting. (Average speed weapons)
1s Primary Attack (animation), 1s Recovery, 1s Secondary Attack, 1s Recovery
The offset is that you actually get an accuracy boost for having a single weapon for not being able to attack as quickly as you can with dual weapons.
Because "Recovery" stat is so important, it is important to note that for each Damage Reduction (DR) your armor has your recovery time is decreased by 5%. So Scale Armor, which has DR 7, gives you a -35% Recovery detriment.