Stack. They multiply each other.
For example Reck + Deathwish. 50% chance of crits and each crit is multiplied by 1.3
Reck + Deathwish + Collossus Smash the same thing with 100% armor reduction. Throw in a Golem'sblood and the 1000 str * king is also more effective.
It's why Fury is aburst spec. Stacking CDs, if done smartly (not in the first few seconds of the fight so tanks can establish threat) is far greater DPS than chaining CDs.