|Darksol the Painbringer|
As with Favored Enemy and Bane, Defiant is only as effective as often as it can be applicable. So if you plan on fighting a lot of X types of creature, getting a Defiant property keyed to that type of creature will ensure a lot of powerful use against that type of creature.
Of course, expecting constant usage is wishful thinking, but there are APs and PFS Scenarios that have the usage of a single type of enemy. (It's practically the sole reason why Crane Wing was viewed as overpowered in PFS, and nerfed because of it, even though it was only powerful due to ignoring pre-requisites.)
To answer your questions now.
1. Yes, you can have 2 of the Defiant properties keyed to 2 different creature types as long as you pay the respective costs and reduce your limits for it. Most properties wouldn't work that way, but since this property has a specific choice that is made at the time of creation, and each choice being (or should be) an independent property of each other, this serves as an exception to that rule.
2. To answer this one, let's refer to the description:
Armor and shields with this special ability excel at blocking the attacks of certain types of creatures, similar to a bane weapon's excelling against certain foes. Against the designated foe, the item's enhancement bonus to AC is +2 better than its actual bonus and provides DR 2/— against attacks from that foe. This increase in enhancement bonus applies only to the armor or shield's enhancement bonus, not to temporary bonuses (such as the magic vestment spell).
Reviewing the text, if you had a +1 Defiant (X) armor and +1 Defiant (X) shield, the Enhancement Bonus increase would stack, as it applies to the enchanted item's enhancement bonus. Since that affects 2 different bonus types, they would stack. However, since both items grant DR 2/- against the designated creature type, that would not stack since that effect is keyed to the same statistic, and share the same type.
Additionally, if you had +1 Defiant (X) and Defiant (Y) Armor, with a similarly enchanted Shield, if a creature fits under both X and Y templates, I believe those effects would accumulate as above. For example, You would have an effective +5 AC on both items against a Chaotic Evil Demon Outsider, though your DR would remain unchanged.