Hmmmm.... interesting. Upon further reflection on the documentation, I feel like there are a lot of misunderstandings concerning flanking.
"When making a melee attack, you get a +2 flanking bonus if your opponent is threatened by a character or creature friendly to you on the opponent’s opposite border or opposite corner.
When in doubt about whether two friendly characters flank an opponent in the middle, trace an imaginary line between the two friendly characters’ centers. If the line passes through opposite borders of the opponent’s space (including corners of those borders), then the opponent is flanked.
Exception: If a flanker takes up more than 1 square, it gets the flanking bonus if any square it occupies counts for flanking.
Only a creature or character that threatens the defender can help an attacker get a flanking bonus.
Creatures with a reach of 0 feet can’t flank an opponent."
My interpretation of that definition is as follows:
Flanking is done in pairs, you must have an ally on the opposite border to flank. If you have a threatening ally on the opposite border, you get to flank. As such, both teammates should receive flanking bonus (you have forced this enemy to haphazzardly defend against two people on opposite sides, so he is easier to hit for both of you.)
As such, with this interpretation, a lone rogue who tumbles behind an enemy should not receive a flanking bonus, as far as I can tell. You need an ally to flank.
Yes? No? Maybe so?