You can rely on what you are told by others so long as you are comfortable with what they told you, but you still have to claim it as your personal knowledge otherwise you can't effectively swear to it. Swearing to something is exactly the same as saying I know this to be true on penalty of perjury. It is a statement to the Court, under oath.
Example:
I submit an affidavit swearing that on June 6, 2011, my office sent a letter to so-and-so which said x, y z. If I wrote and sent the letter, I can swear to it without reservation. If I did not write the letter I can attach it and swear to it because I've seen the letter and the normal business practice in my office is for letters to be sent on the day they are dated by the means identified in the letter. However, if I did not write or send the letter myself, but my colleague tells me that he wrote it and sent it, I can still swear to it (as in, its physically possible for me to do so). But if I don't independently verify that the letter was sent and/or attach it to my affidavit, my affidavit is, at best, deficient and subject to challenge on that basis. Which is exactly the point of O'Mara's questioning.
And you are right that O'Steen also signed the affidavit. So I stand corrected on that point. I've never seen (or at least never noticed) an affidavit signed by two people. Nonetheless, both affiants are independently responsible for the content of the entire document, imo, and Gilbreath is still locked in. I agree with Dershowitz that it is a deficient PCA...period, and I think O'Mara did a great job.