That has BUG written all over it 0_0.
I wouldn't say it's a bug - e.g. if you have +damage on an amulet, you'd expect it to work on both weapons. The point is you can't enchant +damage on an amulet - you might find one with a built-in buff, but for serious +damage enchants you need to cast them yourself, and you can only do that on weapons. So another way of looking at it is that dual wielding doubles the number of potential +damage enchant slots.
But I agree it is generous. On a similar note, +magic damage seems to affect each magic missile, so if you have +200 magic damage, you'll get up to +1,400 damage from a single cast (when you get level 15 magic missile with 7 missiles per cast). Again, very generous, and it makes magic missile much more effective than it's stats would imply compared to other damage spells.
