Backstabbing

A Backstabbing weapon excels at attacking creatures' weak spots. When the wielder is in a position to score a Sneak Attack, a Backstabbing weapon is more accurate and also deals additional points of damage against the foe, even if the wielder has no Rogue levels.

The Backstabbing effect from items and weapons does not stack, only the single highest bonus applies. So, you only need it on one of your weapons or on an item (such as Tharne's Goggles, or the Ninja Spy ToD set).

The Backstabbing bonus will be used for all attacks (single weapon melee, dual weapon melee, and ranged if within 15 meters - the Deepwood Sniper enhancement increases this range to 25 meters), so it can be useful to hold a weapon with a Sneak Attack Bonus +5 in your off-hand when using a thrown weapon.

The same is true of Seeker and Double Strike.

Backstabbing weapons stack with the Rogue Sneak Attack feat bonus, and Rogue enhancements which improve Sneak Attack attack and damage.(source). In game testing also suggests that Halfling Guile enhancements also stack with all of the above.