No matters what i try, it seems that I can't detect if a spell was dual cast or not.

I wanna make a special spell that throws a firebolt when single cast, but a lightning bolt when dual cast.

I added both effects to the spell and tried to add "IsDualCasting" in the conditions but it didn't work, so I tried "EffectWasDualCast" and still nothing.

I also made a perk that reduces magicka cost by 30% when the target is dual casting : I dual cast the spell, hold it, check the spell menu but the magicka cost is still the same.

What bothers me is that usually, I can look at other peoples' work to see how they did certain things that I struggled with, but I have found nothing about dual cast spells.

...As far I know, not counting mods of course, enemy spell casters never dual cast.

So for your perk, the isDualCasting condition needs to be on the Perk Owner instead of the target.