The Zweihanders was used to batter and break up pike formations. They allegedly were capable of cutting off pike heads, however I find this unlikely, or at least militarily unuseful, as even modern swords are incapable of cutting spears without numerous, powerful strokes in the same location- Not easy in the heat of battle.

On plate armor, some zweihanders did have ricasso, or unsharpened sections of blade, which allowed the blade to be grasped and used in a technique called half swording. Half swording allowed for more control and to potentially for gaps in plate armor to be targeted. The existence of ricasso implies that they could be used in this fashion.

Also, keep in mind that the primary function of these weapons and their welders seemed to be to combat pikemen and pike formations, not combat knights. In this period, pikemen would more likely be wearing munition armor which mostly didn't cover the entire body- Some only covered the chest, head, shoulders and thighs.. Full plate was still quite expensive, so despite the massed use of munition armors wouldn't be easily available to the average soldier.