Santa is right.

A random source:
http://wow.allakhazam.com/wiki/avoidance_%28wow%29
Avoidance stacks additively, not multiplicatively. A character with a 50% chance to dodge and a 50% chance to parry will never get hit, not 25% of the time.
But any decent tanking site should tell you the same.

(1 - (0.5 * 0.05) = 0.75 )