Focus effects and stacking ....
Focus effects will stack as long as they don't do the same thing. Regardless of the name of the effect, only the highest modifier will take effect. For example, if you have one focus effect that adds 15% damage to all DD spells and another than adds 30% damage to only cold DD spells, then when you cast a cold DD, you only get the 30% added, rather than 45%. If you cast a fire DD, you get 15% added.
If you have focus effects that add different effects, then they will work together. Like spell haste and mana preservation, etc. You will cast spells faster and save mana.
Much like worn haste, only the highest modifier will be taken into account when a spell is cast.