I use the two weapon combat rules. Haste doubles your attacks, not your OB, so a PC only has his base DB for defense.
For example, if the OB is 100 and the PC haste, he gets two attacks at 100 OB, a net OB of 200. If he parries with 30, he gets two attacks at 70, a net OB of 140. Still in his favor.
Now if the PC is using twc, the net OB gain increases. A twc of 90 parries with 30, leaving 60 OB to attack with FOUR TIMES, for a net OB of 240. Not to shabby.
In core rules, the PC can only parry against the foe he attacks. In my game, parry is applied against all frontal attacks (the three front facings on a hex map).
That is how I do it. I use to do it the second way, which can result in some "Oh crap I have NO DB for parry at all" which makes me laugh but players get nervous.