Well, 2 weapons roughly hit twice in WW, but at their 1h damage.
Since 1h damage x2 is about the same as 2h damage, dual wielding 1h weaps or wielding a 2h weap does just about the same damage.
The exception is grief, whose 1h damage is comparable to the 2h damage of a 2-hander. Thus, dual grief will make more damage than a 2h weapon.