For example 2 CDs, each 10% mitigation.
1. Both seperated:
100-10%=90
100-10%=90
So you mitigate two times 10%.

2. Both stacked:
100-10%-10%=81
100-10%=90-10%=81
So you mitigate one time 19%.

Over the duration of the CDs it is mostly superior to chain CDs instead of stacking.
This counts mostly for teashpulls. On bosses and Savage it can be or is usually likly to stack CDs in order to survive tank busters.