"These weapons are used as a pair, complementing each other. When you attack with a twin weapon, you add a circumstance bonus to the damage roll equal to the weapon’s number of damage dice if you have previously attacked with a different weapon of the same type this turn. The weapons must be of the same type to benefit from this trait, but they don’t need to have the same runes."
So I need to have at least 1, +1 Striking Rune on one of my Sawtooth Sabers and if so that gives me a +2 circumstance bonus to Damage added to the base 1D6 Damage as it is now a 2 for damage dice, correct?
Then as I upgrade the striking rune then +3, +4 and +5 it goes up I'd assume.
Or does this work another way, as I am trying to decide if my Human Rogue will go the Red Mantis Assassin or the Aldori Duelist archtype for the Age of Ashes AP.
You can use entirely non-magical sawtooth sabers; you'll get a +1 circumstance bonus to damage if you attacked with the other saber this turn, since the saber (like every weapon now) does 1 damage die in its own right. If it's got a striking rune on it then you'll get Nd6+N total for N = 2, 3, 4 depending on the rune.
let's say you have a +1 striking and a normal weapon.
with twin it is:
1st attack with magic weapon: +0
1st attack with offhand weapon: +2
2nd attack with magic weapon: +1
2nd attack with offhand weapon: +2
you usually will have the same runes on both weapons (due to doubling rings) so it usually is:
where X is the number of dices you roll.