Most magical buffs don't stack

If a sword is subject to two spells or effects that enhance it similarly, the effects do not stack on each other. Use the better one.
  The same goes with armor. The same goes with people. Unless a spell specifically says "this can stack with _____"
 
  There is a loophole. Unrelated enhancements DO stack. If you have a buff on a sword that increases it's base damage and accuracy AND a spell that wreathes the sword in a coccoon of fire.
 
  Nomenclature can get important when it comes to stacking bonuses. A couple different effects provide "a bonus die to actions", but you can't normally have more than one bonus BUT you can stack a bonus die to actions with an attribute boost. Attribute boosts indirectly provide bonus dice.