Not spear and rapier, shield and spear/rapier. Sorry that wasn't very clear.
Basically, it means you upgrade Stamina like crazy, and then attack from behind your heavy shield. It's cheesy as all hell and doesn't work well against multiple enemies, but it's both reliable and effective.
Of course, you CAN wield a weapon two-handed for greater damage. The stamina consumption is bigger, however, and given that weapons make for a poor defense mechanism, it's not recommend unless you can parry well (in this game, if you time it right and hit R2, you can parry your opponent's strike at virtually null risk to yourself; you can follow with a riposte immediately after that, which is practically an instant-kill - I have yet to see an enemy take a riposte and live, except with a broken weapon).