I think there's an old post around here that does the math, but dual-wielding is pretty much always a better idea than using a shield, since you can block attacks with weapons anyways.
It's like this:
To begin with, a shield is FAR better at blocking than any weapon because they get a 2x multiplier to their block chance. But you can't attack with a shield.
When you gain skill levels with a weapon, you also become more likely to parry an attack with it.
So shields are good for blocking to begin with, but as you gain skill levels with your weapons they begin to outperform shields. If you have two
weapons you are twice as good at blocking attacks, so your chance of blocking an attack while dual wielding is higher than using a single shield once you have enough skill with said weapons.
Some weapons are better at blocking than others too. I wrote this up on the wiki somewhere but I forget where, maybe the weapons page?