Let's say I have two teammates, one with the Splattershot Jr. and one with the N-Zap '85. They both use ink armor around the same time - do I have double the protection for the short duration it overlaps?
No, the defensive power of Ink Armor cannot stack; you either have it, or you don't. However, a second activation will reset the "health" of your armor back to 30 damage points.
I don't know for certain how the duration is affected, but the most likely scenario is that the armor will last until the special gauge of the player who activated it last runs out. Someone ought to test this!