Any Idea if Cast Time Buffs Stack

I have a staff that says it will half casting time of attribute spells 20% of the time twice.

So What is the net effect?

Answsered Perfectly by Nikki Moonlight and The Meth

Thank You

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20 +20 = 40

40%

I can't remember the exact figures, but it's something like 32 percent chance of getting a half cast, and 4 percent chance of getting a quarter cast...

Edit: I was wrong, Meth got it.

The chance of no fast cast is 0.8 * 0.8 = 64%
The chance of 1/2 time is 0.8 * 0.2 * 2 = 32%
The chance of 1/4th time is 0.2 * 0.2 = 4%

For fast recharge there is a cap of 50%, so its a straight 36% chance of fast recharge with 2 20% mods.

I have a staff like this, and I was wondering if there was a chance for an instant cast, does anyone know?

No, there isn't. Just that small chance of a 75% reduction.