MOSFET ORing behaving differently with fast slew rate on one of the Input rails
Those FETs have a gate capacitance of about 3 nF (28 nC to switch). How are your comparators driving them ? to switch in 1 us will require 30 mA of drive current. 

Are your comparators (and references) accurate enough -- is there guaranteed non-overlap between the comparators ? Do you have logic to ensure that the 2nd FET is not turned on until the 1st is actually off ?

User LT4351 instead.