They stack
but additively not multiplicatively if im not mistaken. So a .7 and .9 would give 1.6 not .63 (it would also be really dumb to have a lower armor value for wearing more armor).
To clarify – they don't stack like the way you're thinking. 1.6 would make you take more damage. Armor values are actually multipliers! E.g., 0.9 is 10% damage reduction. :)