So I finally got my character's Illusion maxed and got the Master Illusion spells with eager anticipation with all the Illusion Perks selected.

What I noticed immediately is that the Master spells are automatically set on both your left and right button so they appear to be set up for dual cast.

However, it seems they do not act as dual cast spells, you don't pay double the mana and and you don't get double the effect either. It's not possible to do an authentic dual cast of master level Illusion spells. I did some testing and there were monsters that resisted my Harmony spell (I have all Illusion perks), but when I dual cast Expert level spell Pacify the spell was effective and worked.

This makes zero sense.

In essence this means the following:

- Dual Casting Expert level Illusion spells affect higher level monsters than Master Illusion spells.

- Dual Casting Expert level Illusion spells uses less Magicka than Master Illusion spells.

- Dual Casting Expert level Illusion spells casts faster than Master Illusion spells since there is no additional animation which is important in combat situations.

This is either a huge design flaw or a serious bug as you can't dual cast the Master Illusion spells.

It would also be nice not to have Illusion Master spells throw everything around in rooms - they definitely shouldn't cause physical effects like Destruction spells.

Not sure if this is relevant, but do you have enough magicka to cast the master one twice? If not, try wearing some enchanted illusion spell cost reduction armor. : P