What is the point of having two weapons as a whirlwind barbarian?

Does this mean that a ww-barb with 1 grief phaseblade does as much damage as one with 2-grief phaseblades?

Yes - the damage comes from one weapon at a time. Since you are using the same base weapon with a runeword in it, the only difference between the weapons would be the percent roll you'd get on it.

Wouldn't it be great to use a good shield in the offhand if that is the case?

No, when you block there is an animation, when you are a WW Barb, you want to continuously move and heal through the incoming damage.

It would technically be a waste to add an additional grief, unless you REALLY want that +life and +mana on kill. All of the other stats on grief count when you hit an enemy.

Although expensive, and basically impossible in single player, top end gear WW barbs use a Breath of the Dying 2-H Mace.

I think the discription was wrong. The right mechanic is as follows:

When dual-wielding, both weapons try to score a hit against the target. In other words, the game does a hit-check for each weapon. Since both weapons are given the chance to score a hit, the damage done over time is roughly the sum of the damage of both weapons, which is comparable to the damage done whirling with a two-handed weapon. The effect of weapon speed in dual-wielding Whirlwind is as follows. While a target is in range, one of the weapons' speed is constantly used to calculate the next hit-check frame. If there's no target in range, the game will alternate between weapon speeds to calculate the next hit-check frame. The best way to make sure you have maximum Whirlwind speed when dual-wielding is to use weapons that both reach the final breakpoint, like for instance a Lightsabre phase blade and a colossus blade with 40% IAS.