Practicing longsword fencing, I learned that not only a 2-hander can very well parry a 1-hander, but it can also easily deliver a blow while doing so. Somebody wielding a 1-hander alone against a 2-hander IS at a disadvantage, and a deadly one at that. However, when in close quarters, 1-hander allow much more nimble movements and can feint, bind and lock much more rapidily (and deliver blows from unexpected angles).
The disadvantage at long (out of reach for a 1-handed weapon, in reach of a 2-h) distance can be counterbalanced by the use of a shield (even a small one), a long dagger (or a blade-catcher) or another longer weapon in the left hand. Or, if you feel very lucky, you can try to catch your foe's blade with your bare left hand. The Musée des arts médiévaux de Cluny, in Paris, showed during several years a glove that was mailed on the palm and the inner face of the fingers, just for that. It was dedicated to duels.
How you translate this in house rules for your game is, of course, another story. I did simply state that the parry rules for 2-handed weapons only applied when a foe with a 1-handed weapon managed to close the distance and come toe-to-toe. How to do that? Full parry while still devoting at least 25% of OB to attack, a successful feint or simply do not parry and go full OB while the 2-hander wielder does the same. Risky, difficult, but rewarding.