Think of it like this... You have multiple layers of bonuses:
Layer A: Spell buffs
Layer B: Worn buffs
Layer C: Focus Effects
Layer D: AA effects (innate)
Layer E: Worn2 buffs
Each respective layer stacks with one another but two buff in the same layer "typically" do not stack together.
So with pet bonuses: two focus effects will not stack, but a focus effect and a spell buff will.
Hope that makes sense

Oh the fun you all are going to have in DSK3...
Simon says.. touch the green one! Nooooo BLIQUE NOT THE RED ONE! zomg you just got me "Vaion Touched"!