The Hidden Forest

Most piercing weapons suited for rogues have item properties such as:

Backstab DMG: +38
Skill Mod: Backstab +13%
Extra damages (Backstab): +80

If you have a dagger in your offhand that also has these mods, do they stack?
If so, what would be the compelling reason to use anything besides another dagger in your offand?

I could probably use # mystats to see for myself but I'm not able to log in any time soon.

Not sure if this is 100% correct, but from how I understand it the Extra Damages (Backstab) will stack, up to a point (not sure about the cap, somewhere around 150? I think?). The skill mod will only take the highest +%. The Backstab DMG modifer increases backstab damage... but only counts if that dagger is in the main hand.