/give @p diamond_chestplate[attribute_modifiers=[

{id:"armor",type:"armor",amount:4,operation:"add_value",slot:"chest"}

]]

/give @p diamond_leggings[attribute_modifiers=[

{id:"armor",type:"armor",amount:3,operation:"add_value",slot:"legs"}

]]

When you get two items as, it doesn't add armor, it's added from zero.

In other words, the two do not overlap despite the add_value.

If you wear diamond_chestplate, your armor becomes 4 and you wear it

If you wear diamond_leggings, it's not 7 but 3