Do 20% faster cast items stack?

If i have the 20% faster cast deathly cesta & the 20% faster grim cesta will these stack to faster cast 40% of the time or will it remain at 20%? thanks.

from what ive heard it goes to 32% faster, and also a 4% MUCH faster

its not a stack its more of 2 chances at 20%

i think the math goes like this:

when you cast a spell, there is a 20% chance you will cast faster, with 80% chance normal speed. if you have another item with 20% chance, then out of the 80% chance you cast at normal speed, there is another 20% chance you will cast faster. this leads to:

0.2 + 0.8x0.2 = 0.36 or 36% chance to cast faster.

this implies that if you happen to have 2 items that gives 50% chance, you wont end up with 100%.

I've tested this before and they operate independently. With two 20% faster recharge items, you have a 64% chance of no boost, a 32% chance of half recharge, and a 4% chance of quarter recharge. Note that recharge times are rounded to the nearest integer (except a fractional 0.5 rounds to the nearest even integer).

I have experienced the 'much faster'. I like it.