The chance to roll a 20 on at least one of two rolls of a d20 is 0.1, so equal to the chance to roll a 19 or 20 on a roll of a d20. Using a weapon to give powers radiant so as to gain the benefit of Illuminating attacks from Radiant Servant will pump you up to a 0.2 critical strike chance (1:5!). This does feel OP for something you can gain at level 11, particularly with an execution axe.
Comparing Oath of Enmity to multiple attack powers like twin strike directly is faulty. Even discounting that one has potential broader use, each provides different benefits. Oath of Enmity will max out a single attack and cause critical strike damage once, but with a 2h weapon. Multiple attack powers can potentially cause multiple critical hits, thus dealing critical damage several times, but at the same time they use weapons with smaller hit dice. I'd think OoE provides greater gains on criticals than powers that allow two attacks with 1h weapons, but once you gain a broader crit range the scale most likely tips back to multiple attack powers because of the greater chance for multiple crits.
Comparing Oath of Enmity to multiple attack powers like twin strike directly is faulty. Even discounting that one has potential broader use, each provides different benefits. Oath of Enmity will max out a single attack and cause critical strike damage once, but with a 2h weapon. Multiple attack powers can potentially cause multiple critical hits, thus dealing critical damage several times, but at the same time they use weapons with smaller hit dice. I'd think OoE provides greater gains on criticals than powers that allow two attacks with 1h weapons, but once you gain a broader crit range the scale most likely tips back to multiple attack powers because of the greater chance for multiple crits.