For the purpose of determinig Armor value, you divide any subsequent layer by 2 (round down), be that Ballistic or Impact.
For example, the aformentioned hick Joe-Bob puts on FFBA [4/1], Armored Clothing [3/1], his Armor Jacket [5/3], a helmet [+2/+1], and a set of forearm guards [+0/+1].
His Armor rating would be 10/5 [5 + 4/2 (=2) + 3/2 (=0) + 2 + 1]. Armors with a '+' add their rating directly to Armor without halving. (This takes for granted that you let any armor other than the first count as protection, Some GMs don't because of the ambiguous wording of the rules.)
As for encumberance, it would be 10/6. He would lose a total of 4 Combat Pool dice, and would be at -4 dice to any Quickness-linked skills.