Is it possible to summon multiple zombies? I know there is a perk to get two atronauchs.

And does dual casting help in ANY casting tree except Destruction? If so, how exactly.

dual casting in conjuration makes your summons last longer, not sure if they're more poweful I don't think they are though.
dual casting in illusion makes your spells work on higher level enemies.
not sure about the rest I havnt really played with them.
And yes it is possible to have many zombies with you via the ritual stone which can be found east of Whiterun, between Whiterun and the Valtheim Towers.

the twin souls perk lets you have 2 summons out...

doesn't matter if they're atronach, dremora lord, or raised dead. 2 total.

Max 1 w/o perk

Dual-casting more than doubles the duration of the summon... but the perk for either atronach OR zombie duration (on the way to twin souls perk) is a better buy since you can just 1hand cast it and at less magicka.

Additionally, at 100 skill, you'll start summoning thralls... ie; PERMANENT summons.

With Conjuration you want to pick EITHER Atronachs OR Raised dead.

Play around with both types to see what you like better. Raising the dead has 1 less perk requirement for twin souls. Atronachs can be more versatile if you summon on demand instead of thralling (though still quite versatile) Necromancy can be quite fun because you can build a collection of zombies to use as thralls (re-thrallable when they die too) and store them in a house basemant (in winterhelm)

There are some spells in which you can not duel cast, one of which for an example is transmute. The idea of duel casting is to make a more powerful form of the spell being cast, thus allowing it to be implmented in a many of the spells.

It can increase damage, healing abilties/speed, etc. Just requires a little thiinking out side of the box.