Question about HCT and HSR stacking.

How do multiple instances of Halves Casting Time, and Halves Skill Recharge stack? The Nightbringer has:
* Energy +10
* Cold damage: 11-22 (Requires 9 Death Magic)
* Halves skill recharge of spells (Chance: 20%)
* Halves casting time of Death Magic spells (Chance: 20%)
* Halves casting time of Death Magic spells (Chance: 20%)
* Health +30
* Two-handed
Is it as simple as 20% +20% = 40% chance to halve casting time of death magic, or is there some other formula for it?

IIRC, each chance is applied separately. In other words, the game first checks the first 20% chance, then it checks the second 20% chance - even if the first check succeeded. That means that, in a 20-20 situation, you have:

.8*.8 = 64% chance of normal cast
.2*.8*2 = 32% chance of 1/2 cast time
.2*.2 = 4% chance of 1/4 cast time

However, stacked HSRs can only reduce recharge time to 50%, so if both activate you'll still just get half recharge time. HCT doesn't have a limit on how low it can reduce cast time.