1) Not a chance. Benefits from the same source do not stack AND Shield bonuses do not stack with shield bonuses.
2) Not a chance. Benefits do not stack from the same source.
- Gauss
Let's look at the RAW regarding Shield of Swings...
When you take a full-attack action while wielding a two-handed weapon, you can choose to reduce the damage by 1/2 to gain a +4 shield bonus to AC and CMD until the beginning of your next turn. The reduction in damage applies until the beginning of your next turn.
It appears this benefit only works once, since the key note is that this triggers when you take a Full Attack Action to gain that +4 shield bonus to AC (and CMD, which is not normal for CMD bonuses, so it's still nice in that regard). However, it's not that bad, especially considering that this should only reduce the damage for one of the Greatswords that creature is wielding, since the penalty only seems to apply to a single weapon.
In addition, I doubt 2 shield bonuses from the same source would stack, meaning no +16 even if it were not based on a Full Attack Option.
Now, let's look at the RAW regarding the Defending Property...
A defending weapon allows the wielder to transfer some or all of the weapon's enhancement bonus to his AC as a bonus that stacks with all others. As a free action, the wielder chooses how to allocate the weapon's enhancement bonus at the start of his turn before using the weapon, and the bonus to AC lasts until his next turn.
The benefit it grants notes that it transfers an AC bonus up to the weapon's enhancement modifier, and that it stacks with all others; this would include other Defending Weapons. So yes, they would stack for a +20.
However, following the URL posted above, there is an important Errata/FAQ that is listed, and I suggest that you read it.
If you're lazy, I will summarize it by saying that in order to get the benefit from the Defending Property, you must make an Attack with each of those weapons in order to get their benefits; otherwise they do not work.