Carrying two weapons no longer gives you two attacks except in the case of some Ranger powers. Carrying two weapons is now mostly a special effect.
There are no penalties or benefits to using two weapons other than being able to use either weapon when you attack. There are also a couple of feats that allow you to take advantage of the other weapon and give yourself +1 to damage and +1 to AC/REF when you dual wield.
Because of the changes in two-weapon wielding, there is no more need to consider a staff a double weapon.