Hey, I have one question 
I do not understand this part in text; 'off-hand property'
When i wield two drow long knife, if i take this feat my bonus damage increase +2 (both of them)? And do i get - penalty attack rolls for this ?

Tempest Technique said:When wearing light armor or chainmail, you gain
a +1 bonus to damage rolls with melee and close
weapon attacks when you are wielding two weapons.
This bonus increases to +2 with weapons that have
the off-hand property.
I do not understand this part in text; 'off-hand property'
Two-Blade Warrior
[Multiclass Ranger]
Prerequisites: Str 13, Dex 13
Benefit: You gain training in one skill from the
ranger’s class skill list.
You can wield a one-handed weapon in your off
hand as though it were an off-hand weapon.
When i wield two drow long knife, if i take this feat my bonus damage increase +2 (both of them)? And do i get - penalty attack rolls for this ?