Hey Siobhan, thanks for the answer. I realised the complete edition (Drakensang+Am Fluss der Zeit) I bought comes with a pdf version of the rules (second printing, in 2008, I think it has to be the 4.1th edition) and it does say that once a shield is equipped, the weapon PA value won't ever be taken into account. Actually, I found a digital version of Wege des Schwerts and it seems that Drakensang uses a simplified version of the core ruleset with handpicked additions from the extensions. For instance, the special skill Linkhand doesn't exist, and neither does the Meisterparade maneuver. In the videogame adaptation, there is no option to convert an attack into a parry, etc.
Even if there is no way to check how exactly the developers implemented the rules (in-game, the console gives feedback about attack rolls only), I think you're right, there is no combination of weapon/shield parry.