GURPSよろず

Parrying with Two-Handed Weapons
Many great Masters of Defence wrote of the virtues of long, two-handed weapons – notably bills, greatswords, bastard swords, and quarterstaffs – for fending off multiple adversaries. The heft and surface area of such weapons do make it feasible for a skilled warrior to angle his weapon to stop several attacks in rapid succession. Usually, he wards off distant blows in a manner that obstructs close-range ones, or trades reach for time by dealing with progressively more distant attackers one at a time as they approach.
If the GM decides that such claims are true in his campaign, he might allow any ready Polearm, Spear, Staff, or Two-Handed Sword weapon that’s at least two yards long and wielded in two hands to use the following rules:

Dual-Weapon Parry: A weapon like this can make a single parry at -1 to deflect both halves of a Dual-Weapon Attack (p. B417). Success wards off the two attacks. Critical success means the attacker must roll on the Critical Miss Table (p. B556) once for each weapon. On any failure, though, both blows hit!
Multiple Parries: These weapons have half the usual penalties for parrying more than once in a turn (p. B376). Thus, successive parries in a turn are at a cumulative -2 per parry after the first – halved to -1 per parry for a Weapon Master! A Dual-Weapon Parry counts as a single parry for this purpose.