Some elite bosses can resist this effect, and only take normal damage from death cloud.
You can easily unlock the "Heavy Hitter" achievement with this spell combination.
Death Hex is dispelled by applying the combination while Death Cloud remains active. Thus you can also hit a single target multiple times provided you have more than one mage able to cast Death Hex in your party. Resetting the spell timer also works.
If both spells are cast by different mages, the damage is determined by the spellpower of the one that cast Death Cloud. The kill will also be assigned to him/her.