Does this even do anything? In active effects it still shows it as having the same effect as using one ward.

I would venture to guess it doubles the effect?

I don't know, I've never dual cast wards before, but it seems that "doubling" is a pretty good guess when looking at effects (I.E.: Conjuration dual casts double duration, "Shield" spell dual casts double duration, etc.)

You can actually see that there is a difference when you dual-cast other spells (60 sec duration becomes 132 sec when dual-cast or something). Dual-casting a ward just shows the same active effect as when you cast one ward. Using two wards (not dual-cast) shows the ward effect two times tho.

Hmm... I wonder if maybe it's a bug?

Tested it, and the ward didn't get any stronger. Gave an enemy a staff that did 60 frost damage and I used a Steadfast ward to test. Each hit stopped the ward without stagger, whether it was 1 ward, 2, or a dual-cast one; the effect didn't change.

It makes it twice as powerful?

I don't think it works with wards