Quick question for you illusionists out there: Does the dual-casting perk in the illusion tree just increase the duration of a given illusion spell, or does it allow it to work on more powerful enemies?

I suspect the former, but my Googling has turned up contradictory information and I want to be sure.

And in any event, do you think the perk is worth it? I'm playing my first illusion mage.

They were at least smart enough to make dual-cast for Illusion affect magnitude. It is an indispensible way to give illusion spells a lot more oomph.

So by "magnitude," I take it you mean that a dual-cast version of a illusion spell can work on an enemy that would otherwise resist the basic version of the spell? If so, that's very cool, and sounds like it's definitely worth the investment--more so than dual-casting perks in other trees (i.e., destruction) which only give you a small increase in power for a significant increase in magicka cost.