The conditions Chilled and Crippled both reduce the movement speed of the target (by 66% and 50% respectively). Does the movement speed reduction for each skill apply, or does the movement speed from only one of the conditions apply? In other words, if a target is inflicted with both Chilled and Crippled, what will be their total reduced movement speed?
Both of the links you provide say "Stacks in duration". According to the Effect Stacking page
It also says
Originally I interpreted this to say higher damage trumps lesser damage but increases duration which is incorrect. Based on info I found at Guild Wars Insider the answer becomes a little more complicated.
Multiple duration effects are applied in sequence for their applicable lengths of time. In this example lets say Chilled got applied then Crippled within 10 seconds of each other and both effects are for 60 seconds in length. Chilled would get applied to you for 66% reduction, Crippled would wait until Chilled had finished then immediately get applied to you for the 50% reduction for another 60 seconds.
The blog post at Guild Wars Insider has a nice graphic to show how this stacking occurs if my explanation is confusing.