Dual wielding is for Berzerkers. They know not what blocking is.
Not true.
I could role play as some Samurai-like sword master who uses two katanas.
Anyway, I really hope that we can block while dual-weilding.
I really see no reason not to.
Balance?
Nope, it ain't it!
I've already said that it could be balanced easily if it's required.
With shield, you could block power attacks, normal attacks, projectiles and maybe even magic to some degree.
With dual-weild, you cold block only normal attack and only partially block power attacks.
And let's not forget that shield users can bash with their shield also.
Command for blocking while having two weapons equipped would be pressing (and maybe holding) left and right keys at the same time (which also makes it harder to block than with shield).