mitigation in this game stacks multiplicatively. So using rampart and reprisal means youll end up with a 28% damage reduction instead of a 30% damage reduction. The diminishing returns are extremely dependent on the combination used, but you can check it with simple math (1-a)*(1-b).
This means that technically, you get more out of each mitigation cooldown if you use them on their own, however, in high end content several attacks will require multiple mitigations to be used to survive an attack, so stacking is sometimes inevitable. In dungeons, trash packs often die before you use every single cooldown, so stacking one or two them is fine as well. Just keep in mind that the more cooldowns you stack, the less effective the next one becomes.