The official ruling on record is that you *can* change OB/DB split if you get a second attack
The problem with using the term "Parrying" is that most people take that to mean that it is ONLY deflecting an incoming attack with your weapon -- and that just isn't the case. "Parrying" should more accurately be called your "Combat Posture" as it reflects more how defensively you fight overall.
[oRule=Official Ruling: OB/DB Split & Multiple Attacks]
When a player declares the OB/DB split for the round, he is declaring his overall defensive posture. The amount of OB moved to DB reduces the character's effective OB for the entire round. If the character is able to make multiple attacks, ALL those attacks will utilize that "effective OB". The player does get to make multiple OB/DB split declarations for each attack.
It does not matter if the character gains multiple attacks from Two Weapon Combo/Combat (however you call it), or through having enough activity percentage available through Haste. This ruling applies to all of them equally.
Additional Note: Since "Parrying" is basically your combat posture for the round, being disarmed will NOT remove the modifier to DB gained from Parrying. The same applies to critical results that also disarm the character or that destroys their shield.
It is also important to remember that the "attack roll" isn't actually a single roll, but the cumulative effect of numerous lesser attacks all rolled up into one, in a single roll. The "initiative order" in which events happen is simply a device to determine when the GM and players resolve the actions, and is not actually indicative of when those actions actually occurred within the round.
[/oRule]