Spiked armor is worn, thus you can't duel wield with it.
When I first read this my mind automatically replaced duel with dual, so I thought that there was actually a rules clarification stating that you cannot wear two sets of armour at the same time. But I guess a dwarf does it in Boarmurdered so you can't argue that there isn't precedence for it.
This is another instance where all of these rulings conform to my current understanding of the rules so no shockers for me.