Multiple percentages stacking?

I have a "Forget Me Not" and an "Aptitude Not Attitude" on my elementalist's wand and focus, but have the chance to add a "Wand Wrapping of Memory" that does the same thing as the inscription. Will these "stack" int he sense that I have more of a chance to halve the casting time or recharge of a spell, or will it remain at 20% from my inscription? If they will stack and the wrapping is tested for a 20% chance and then the inscription gets checked in the event that the wrapping didn't go off, I will add the wrapping. Thanks for the info!

That's how it works. You effectively get a 38% chance of HSR if you have 2 20% HSR mods.

Yes they stack. With a 40/40 set you even have the chance for both half casts to work at the same time. i.e. double HCT 1/2 second fire attunement. I haven't noticed this for HSR though.

Awesome, thanks for the info!

The correct answer is:

Dual 20% half cast/recharge time mods results in:
4% of 1/4 cast/recharge time,
32% chance of 1/2 cast/recharge time, and
64% chance of normal cast/recharge time.

However, remember that recharge time is capped at 50% faster. So that 4% of 1/4 recharge time is going to hit the cap and get taken down to 1/2 recharge time, except in rare cases where you've got something slowing your recharge time.

(There is no limit on faster cast.)

[edit: corrected idiot typo]