Issue with using 2 daggers, you won't have a high base damage compared to dual wielding a dagger + cestus/club.
This is due to not having legendary achievements for 2 weapons (a loss of 7.5% p-attack bonus) and daggers having a very low base attack.
As far as the Dagger mastery is concerned, I didn't bother with it since Dagger's have low base attack anyways and I didn't see a severe change in other builds with weapons that have high base attack (ie katana/purple cestus) so I figure 15 points into that is useless. I could use those points for Eva/Crit rate or even crit dmg to round off what legendary achievements/glyphs and enchants wont cover.
And Im pretty confused about the magic dmg comment so no comment on that.