Omg people THAT is realy common knowledge!
Simply: IF a Spell has casttime under 1,5 sek you cantcast instantly another after because of the Clobal Cooldown.
So, if you cast longer than 1,5 Sek the Global Cooldown is finished and you can directly hit with an INSTANT Spell after the Castet one and maybe before the first one hit the target.
I Use this since 3 years... it´s nothing new and simply game mechanic.
Btw: That can be bad if you cast something at a Shaman and he has this absorb totem out, if you do this doublehit both spells are "away" because they hit at same moment often.
ps: i know my english isn´t the best....