It is not listed if the two spells stack, at least not in the rulebook. Can you have both active? Spells linked below:

Mage Plate
Incant: "I invoke Protection to grant Mage Plate."
Duration: 5 days or until used / destroyed
Description: This spell will surround the caster in semi-transparent plate armour. This plate is Magical in nature and may be Dispelled as normal. Mage Plate grants the caster +30 Magical Armour Points. This bonus armour will not stack with the Mage Robe or Mage Leathers spells but may stack with any other armour the caster is wearing. All lightning based attacks that strike the Mage Plate do double damage. This spell is caster only.

Barkskin *
Incant: "I invoke Nature to grant Barkskin."
Duration: 5 days or until used
Description: This spell gives the target 40 points of Magical Amour. This armour is Magical in nature, always comes off the target first, and may not be Refit. All fire based attacks that strike the Barkskin do double damage. If the armour is Breached the remaining fire damage will carry over to the target.