It's unfortunate that in a heavily pirate themed game, the Swashbuckler fantasy of melee/ranged is inferior to melee/melee, ranged/ranged. This is because Full Attack abilities, which will hit with both weapons, only hit with the primary weapon if you mix weapon types, an effective -50% damage. Disappointing, considering the clearly intended way to use Scordeo's Saber/Pistol.
However, there actually does exist a way to Full Attack with both a melee and ranged weapon. It's the Fighter cleaving/mob stances. If you wear a melee and ranged weapon and kill a target in melee range, both weapons with fire on a nearby enemy.
Seeing this, I immediately thought to mod this so Full Attacks finally works. However, in reality it isn't that simple. Here's how Cleaving stance work:
1. Cleaving stance apply a status effect on you.
2. The status effect check if you kill something in melee range.
3. An AoE pulse attack is triggered, applying a status effect on a target in melee.
4. The target receive a Full Attack.
Why does the Full Attack from cleaving/mob stance trigger attacks with both melee/ranged weapons, while normal Full Attacks from abilities only trigger the primary weapon? I have no idea.
Here are the problems: theoretically it should be possible to add a targeted pulse to trigger a secondary attack on Full Attack abilities to simulate a Full Attack with melee/ranged. However, this would trigger regardless of weapons so melee/melee would hit thrice.
Another way could be to replace the attacks on abilities with a Full Attack pulse. This would however change the abilities because they would be instant with no attack animation/casting time. It would probably also break Ranged Full Attacks.
Basically, I haven't managed to make it work, so I'm documenting it here for posterity.

Edited December 17, 20223 yr by RilleL

i apologize for the lazy answer but if I remember correctly, the abilities are often weapon-attack-abilities which are sort of hybrid "abilities"+"attacks" and so because cleaving/mob stance uses full attack via "Status Effect" it forces attacks from both weapons regardless of range.

I guess it's something like that. I can confirm that some active abilities does trigger a full melee/ranged attack, like Barbarian's Heart of Fury and Monk's Flagellant's Path. I haven't checked the game-files for how they work though. I guess a lazy solution is to avoid abilities which does not trigger both attacks, like Barbaric Blow. I really wish Obsidian had made Full Attacks consistent across all abilities.

Whirling Strikes too. 

Do these Full Attacks receive a malus when dual wielding ? I would presume no, but I haven't checked.