How does Taal's Twinned Arrow work?

The Chaos Wastes game mode boon, "Taal's Twinned Arrow" is described:

Ranged attacks now fire one additional projectile.

How does this work with weapons and career skills / ults where this description is not quite straightforward, like ranged attacks that don't have projectiles or have multiple projectiles per shot? For example:

• Sienna's Beam Staff - How are its beam, blast, and burst attacks affected?
• Sienna's Conflagration Staff - I noticed that it works as described with the primary fireball attack but I'm not sure how it affects the secondary area of effect attack.
• Sienna's Coruscation Staff - How are the shotgun blast-like primary attack and the secondary area of effect attack affected?
• Flamethrower-type weapons: Sienna's Flamestorm Staff and Bardin's Drakegun - How are the flames affected since they're not exactly "projectiles"?
• Sienna's staves in general: Does it also generate overcharge for the additional projectile fired?
• Shotgun-type weapons: Bardin's Grudge-Raker and Kruber's Blunderbuss - one extra pellet or double the total pellets per shot?
• Are the projectiles of ranged attack career skills / ults (e.g., Waystalker, Bounty Hunter, Pyromancer, Outcast Engineer) affected? How are they and their variations as modified by talents affected?
• Saltzpyre's Rapier special attack (pistol shot). It is not exactly classified as a ranged weapon in the game, so I'm not sure if it's also affected.