Bucklers provide shield bonuses to AC. If you have two, you have two shield bonuses to AC, and as typed bonuses they do not stack—you use the higher one.
Any penalties, though, do stack. That means the attack penalties, the armor check penalties, and so on. The arcane spell failure would stack as well, though if both are mithral, 0% + 0% = 0%, so you would still have none.
For the purposes of other bonuses from special properties, though, I see no reason you couldn’t use two. But I would definitely ask the DM to make sure he or she is OK with it because it is kind of unusual.
No (but your DM can allow it)
Player's Handbook, p.144:
You can benefit from only one shield at a time.
It doesn't say you only gain the AC bonus from one shield, it says you cannot benefit from more than one shield. If you attach a second shield, only one of your shields has effect (unless a more specific rule elsewhere overrides that).
Note that it doesn't say which of your two shields has effect, if you've attached two. You could, arguably, swap between them ambidextrously as benefits you, but there's no particular rule covering this rare case.
Dungeon Master's Guide, p.141:
Multiple Items of the Same Kind
Use common sense to determine whether more than one of a given kind of magic item can be worn. A character can't normally wear more than one pair of footwear, one pair of gloves or gauntlets, one pair of bracers, one suit of armor, one item of headwear, and one cloak.
This doesn't explicitly describe shields, so it doesn't override the general rule that you can only benefit from one shield at a time. The fact that it doesn't say "you can only use one shield" doesn't implicitly mean that you can use two shields, because the general rule for shields is that there can be only one.
However, it does advise the DM to "use common sense", which means that it's up to the DM whether or not you can actually wear and benefit from two shields.
Best Answer
The Aegis of the Raven Queen states:
That's your normal shield bonus of +2 for using a shield, plus another +3 from that particular shield's magic, for a total of +5. You don't have to use a second shield, nor should you.
You can wield two shields at once, but only the better one would apply to your AC, so there's usually no point in wielding twin shields. The relevant rule in the shield description simply says (emphasis mine):