I've played some swashbuckly types in our games. We've handled it as follows:
A main-gauche is basically a long dagger, often with an extra-wide cross-piece, for parrying. When being used to attack, we use the standard stats for a dagger.
Use the Off-Hand Parry feat in Sword and Fist to replicate the parrying. It increases AC.
We started with some "active parrying" (before S&F came out), similar to 2E, where you make a roll against the attacker's to attempt to "beat" them and parry the attack. It just got clumsy and at higher levels, with more attacks, bogged down the action.
You could also, I suppose, come up with some other feats to duplicate things you can do when you have a parrying weapon and an attack weapon. Weapon locks (perhaps leading into disarms or even sunders). Ripostes. Etc.