Actually it's "whenever you roll the same number on your two oath dice, as long it's a hit, you crit"* Take the Ardent Champion paragon path. The feature "Holy Ardor" makes it so that whenever you roll the same number on your two oath dice, as long as they're not 1s, you crit.[/IMG]
A double 1 is the most obvious miss, but if you need a 10 to hit, a double 9 is still a miss and thus not a crit.