Glancing Blows variant
Under this variant, attacks use two attack rolls instead of one, but have no damage rolls.
If both attack rolls hit, the attack hits and deals maximum damage. This is referred to as a solid hit.
If one attack roll hits but not the other, the attack hits and deals minimum damage. This is referred to as a glancing hit.
If neither attack rolls hit, the attack is a miss and deals no damage.
The damage for both solid and glancing hits is maximized or minimized entirely, including all bonus damage dice.
Critical hits, I'm not sure about. Here's a quick thing, but it doesn't feel right and might be too complex.
Critical hits occur normally, and are threatened based on the higher attack roll's result, but are only confirmed if the confirmation roll is a solid hit. The extra damage from a critical hit is rolled normally (whether it triggers on a glancing or solid hit).
Feats:
Precise Strikes: You can take a penalty to your damage (up to a maximum of your BaB) to gain an equal bonus on your lower attack roll.
What does this mean?
At lower levels, when damage is high and accuracy is low, sudden lethal blows are less common, but weaker ones are more frequent. Battles are less of a coin toss about who gets a lucky blow in first since glancing blows can more easily wear an opponent down.
Static damage modifiers are more important when your accuracy is low (yay, Power Attack), but bonus damage dice are more valuable when accuracy is high (yay, Sneak Attack).
So, there are still a few kinks to work out.
