Option : Use the Estock!! Arms Companion. It is now my favorite weapon and it does Puncture Crits!
Who doesn't love a good puncture crit?
As for the Parrying and the OB, it comes down to what is left over when you make your attack. You still get to make an attack at +0 OB if you go full parry and which attack table you are going to resolve the attack on.
A) If you want to go full parry with 100% OB, that implies you've used the sword as a 1H weapon and therefore resolve on the 1H attack table with +0 OB
B) If you want to attack on the 2H attack table, then you can only parry at 50% and therefore have +50% OB to your attack. You don't get to have your cake and eat it too; full 100% DB but resolve damage on 2H table.
It's not so much a discussion of DB and parrying skills and sphere influence, etc. It's what the intent is. Even if you are using a 2H sword and you want the 100% parry bonus to DB, then whatever is leftover is going to be pathetically weak as the 2H is by the very nature, heavier and longer, whatever. If you've committed yourself to a full out defense with this weapon then want to pull off an attack as well, even if you hit, it's won't be as effective and would probably do considerably less damage if you connected.
(i.e.: It would probably hit with effectiveness of a 1H sword with a +0 OB rather than a 2H weapon. At the very least, same penalties of wielding a 2H weapon 1H, I would assume since you are trying to parry with it as if it were a 1H weapon.)
Long 2H weapon vs. Shorter 1H weapon: This is directed more at the sphere of influence. It would appear that a longer weapon is better at parrying, but as BHanson stated, it's not so much parrying as it is controlling a sphere of influence. The 1H (or even paired weapons) are lighter and quicker allowing the wielder to maneuver quicker. Once the 1H opponent is within the effective range of the 2H weapon (probably from a successful parry of his own), parrying is extremely difficult. The 2H cannot move as quickly to counter the attacks. The wielder can't use 1 hand to fend off or redirect an attack as that requires letting go of the 2H weapon and trying to wield it one-handed.
Look at a battle between a 2H sword and a 1H and shield. Heck, watch The Holy Grail. 2H weapons are heavier, easier to see the attack coming, harder to make a follow up attack, and once the opponent is inside the attack range, very difficult to use for offense or defense.
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This is just a side note and I don't think this is a house rule, but weapons that are able to be wielded 1H or 2H did not require spending double the points or to be treated as a second skill. The weapon was designed with that intent in mind. It has a longer handle, it has lighter material, the blade is thinner, there is a dulled section on the blade meant to be used a an extra hand-hold. a la Conan's hand and a half sword. We viewed the ability to switch at will as parrying ability, an extra OB *if* used as a 2H weapon, ability to use a shield but resolve attacks on the Broadsword table, etc.
Look at Hurn's sig.
'Last of all, Húrin stood alone. Then he cast aside his shield, and wielded an axe two-handed'. --J.R.R. TolkienThat doesn't mean that Hurin developed a second skill with Axe - 2H. It means that he had an extra bit of grip so he could swing harder, do more damage, at the expense of DB and loss of the shield.