If you have 2 custom peices of armor each with their own custom Armor attribute they do not stack. For example if you have a chestplate with +2 Armor and leggings with +2 Armor they do not add up to be +4, it stays at +2.

What I expected to happen was...:
Have +4 Armor with both pieces of +2 armor equipped

What actually happened was...:
I only gained +2 armor

Steps to Reproduce:
1. Execute both commands:

/give @p minecraft:leather_leggings 1 0 {AttributeModifiers:[

{Slot:"legs",AttributeName:"generic.armor",Name:"generic.armor",Amount:2,Operation:0,UUIDLeast:41580,UUIDMost:573197}

]}

/give @p minecraft:leather_chestplate 1 0 {AttributeModifiers:[

{Slot:"torso",AttributeName:"generic.armor",Name:"generic.armor",Amount:2,Operation:0,UUIDLeast:41580,UUIDMost:573197}

]}

2. Put both pieces of armor on your player

3. See what happens