The way I read it you get one "set" of Defensive from weapons, whether you're using one or two (and a shield counts as a weapon) and one "set" from the armour you're wearing. Talents that give +1 Defence (Superior Reflexes I believe it's called) also stacks, unless otherwise specified such as with the Defensive Training talent.
Well there's Melee Defense and Ranged Defense (the Defensive & Deflection qualities can add to these), and then there's General Defense. General Defense does not stack with Melee or Ranged Defense.
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Armor increases General Defense, meaning that it doesn't stack with Defensive or Deflection qualities.
The talents Superior Reflexes and and Sixth Sense offer +1 Melee Defense and +1 Ranged Defense (not the Defensive or Deflection qualities), so those don't stack with armor either. The way I read them, though, is that they would stack with the defense offered by the Defensive & Deflection weapon qualities because of their "+" symbol.
I see it as General giving you a base rating in both Melee Defense and Ranged Defense which may be modified by equipment or talents if they modify the vaule ("increases by", "+1 X Defense") instead of setting it to set number (Static Cover Bonuses).
Someone should send it to the developers for clarification since there seem to be several conflicting interpertations of the rules about defense.