Can you wear a Mithral Chain over Snakeskin Tunic and have their effect stack?

Question

Can you wear a Mithral Chain over Snakeskin Tunic and have their effect stack?

Snakeskin Tunic: A snakeskin tunic is a tight, form-fitting shirt crafted from the scales of a giant snake. When worn, it grants a +1 armor bonus to AC, a +2 enhancement bonus to Dexterity, and a +2 resistance bonus on saving throws against poison.

Mithral Shirt: This extremely light chain shirt is made of very fine mithral links. The armor has an arcane spell failure chance of 10%, a maximum Dexterity bonus of +6, and no armor check penalty. It is considered light armor.

Answer 1

Armor bonus no, other effects yes.

Armor bonuses to AC do not stack.

As listed in the glossary of common terms:

Armor

An armor bonus applies to armor class and is granted by armor or by a spell or magical effect that mimics armor. Armor bonuses stack with all other bonuses to armor class (even with natural armor bonuses) except other armor bonuses.

And as listed in equipment:

Armor/Shield Bonus

Each type of armor grants an armor bonus to AC, while shields grant a shield bonus to AC. The armor bonus from a suit of armor doesn't stack with other effects or items that grant an armor bonus. Similarly, the shield bonus from a shield doesn't stack with other effects that grant a shield bonus.

As for the other bonuses and effects of the two items, as they are items in different slots (armor slot and chest slot, respectively) with different bonuses they should stack just fine, since, unlike Bracers of Armor, Snakeskin Tunic does not have a clause that turns off its other abilities when another armor bonus takes precedence.

Magic Items Slots

Many magic items need to be donned by a character who wants to employ them or benefit from their abilities. It’s possible for a creature with a humanoid-shaped body to wear as many as 15 magic items at the same time. However, each of those items must be worn on (or over) a particular part of the body, known as a “slot.”

A humanoid-shaped body can be decked out in magic gear consisting of one item from each of the following groups, keyed to which slot on the body the item is worn.

[...]

Of course, a character may carry or possess as many items of the same type as he wishes. However, additional items beyond those in the slots listed above have no effect.

Answer 2

The effects do not stack, they overlap.

Named bonuses very rarely stack with themselves (see Bonuses). The +1 Armor bonus to AC from the Snakeskin Tunic and the +4 Armor bonus to AC from the Mithral Shirt would not stack. Instead, they would overlap, which means that only the highest applies (in this case, that's clearly the +4 from the Mithral Shirt).

If the tunic provided (say) a +1 Natural Armor bonus to AC, it would stack with the Mithral Shirt.

However, the tunic's other effects may still be present: barring other items with similar bonuses, the tunic's +2 enhancement bonus to Dex would still apply as would the +2 resistance bonus to saves vs. poison. Of course, the tunic's +2 enhancement bonus to Dex would overlap with (say) a Belt of Incredible Dexterity - only the higher bonus would apply.

Answer 3

Two armor bonuses does not stack, everything else will

Bonuses with the same name or from the same source do not stack, as explained on the glossary:

Bonuses are numerical values that are added to checks and statistical scores. Most bonuses have a type, and as a general rule, bonuses of the same type are not cumulative (do not “stack”)—only the greater bonus granted applies.

The important aspect of bonus types is that two bonuses of the same type don’t generally stack. With the exception of dodge bonuses, most circumstance bonuses, and racial bonuses, only the better bonus of a given type works. Bonuses without a type always stack, unless they are from the same source.

That means two armor bonuses will not stack, only the highest will be applied, two natural armor bonuses will not stack, two deflection bonuses will not stack, and so on.

If you look at this image from the SRD, you will see that the Snakeskin Tunic uses the Chest magic item slot, while the Mithral Shirt uses the Armor magic item slot.

Each slot can only gain the benefits of a single magic item, but your character will benefit from all magic items from all those slots at once, assuming the other rules are respected (like stacking bonuses).

When a character wears a slotted wondrous item he cannot gain the benefit from a wondrous items of the same slot until the first item is removed. Of course, a character may carry or possess any number of slotted items of the same type, but additional items have no effect until they are worn.

In other words, your character will gain the benefit of both magic items (except for the armor bonus).