Actually, the rule specifically states
"Every type of weapon has an Armour piercing (AP) score. The character weilding the weapon adds his strength modifier to this AP, if he is able to add his strength modifer or some multiple of it to his damage roll with the weapon."
So, whether your damage modifer is STR MOD x 0.5, STR MOD, or STR MOD x1.5, you only add your base STR MOD to the AP.
Good news for two-weapon fighters.
Bear in mind, the two-handed weapons generally already have high AP modifiers, and the light weapons generally already have low AP mods, so the "two-handedness/offhanded-ness" has already been applied to the AP.