Destruction has stagger, but what about the other schools?

Restoration

Illusion is pretty useful with dual casting as well.

Conjuration maybe. Duel casting a spell makes whatever you summoned last longer.

Though that would be useless for dead thralls or any of the thrall spells.

They're two handed spells anyway.

Dead Thrall is a 2h spell to begin with, along with any master spells.

Yeah, but more than doubling the duration for one perk point makes the journey from 20 to 90 Conjuration skill much smoother. Same thing with Alteration.

For Illusion it's practically a "must have" due to the level gap between the weaker and stronger versions of the Fury/Calm/Fear spells.

For Restoration it's rather useless. You don't get enough benefit for the extra magicka it costs to Dual Cast as opposed to just casting two of the same spell.

I never use thralls. They lag behind just like other followers and have to be resurrected when they get killed. It's so much easier to just conjure a helper into the middle of a fight or whenever they are needed. But I never dual cast them either because they usually don't last long enough to bother with that anyway.

Only thing I dual cast is something like Frenzy when it won't work on a higher level enemy. I don't dual cast destruction because I'm usually tossing that from one hand while using something from another school from the other hand. It's more fun to mix it up. Stun locks are boring as hell.

Never really used master spells so thanks.

Destruction, Alteration and Illusion are the ones that give you the most bang for your buck as far a spent perks go.