OK, here's what I already know about magika duel wield in update 13:
DW now gives the same base spell damage as a 2 handed weapon, which is still more than a staff or bow.
DW still has the advantage of an extra set piece at the expense of being able to weave attacks.
DW was DPS tested by a magplar on PTS and still slightly outperformed both lightning and inferno staves.
Here's what I don't know:
1/ Does the "duel wield expert" passive provide any benefit to magika users?
2/ Does the "twin blade and blunt" passive provide any benefit to magika users?
3/ Are swords still better than daggers, axes and maces for magika users?