I'd say that the sheer force of a twohanded blow against the head is going to at least daze you quite a bit.
But in defense of armour...
I assume Two-handed swords (used in a classic fantasy-style, also M&B-style) were not particularly effective against plate armour. At least fencing manuals for using the longsword (from the 13-16th century) describe specific techniques on how to deal with armoured opponents. One is seizing the blade with one hand, to get more precision for stabbing and slashing at the joints (http://www.thearma.org/pdf/th11.jpg).
Another is to simply turn the sword around to use the handguard as a pickaxe, to focus as much force as possible on one small spot (http://www.thearma.org/pdf/th10.jpg).
http://www.thearma.org/manuals.htm