What I expected to happen:
My health would add up to 30 health when both the helmet and the chestplate were worn.

What actually happened:
My health stayed at 25 regardless of the fact that I was wearing both the helmet and the chestplate.

Notice how in the screen shots, the health is always 25 no matter what combination of armour I wear.

Commands used:

/give @p iron_helmet 1 0 {AttributeModifiers:[

{Operation:0,UUIDLeast:3,UUIDMost:3,Amount:5.0,AttributeName:generic.maxHealth,Name:Helmet}

]}

/give @p iron_chestplate 1 0 {AttributeModifiers:[

{Operation:0,UUIDLeast:3,UUIDMost:3,Amount:5.0,AttributeName:generic.maxHealth,Name:Chestplate}

]}