Well, I know that the other crit chance mods do stack. That is, I have two items which each has a +15% crit chance mod. Those stack. So I would suppose that the deadeye mod stacks. Can you put Deadeye in other places than helmets, though?
Edit: I just had the opportunity to test some stacking, and can confirm that the 30% crit damage mods only stack once. So I suppose that the limit is that a mod can only stack once.
Further edit: Same goes for crit chance mod. That also only staks once. So the question is, if it's the mod or the bonus which can only stack once. So that you can in reality use no more than two mods which gives bonus to something.