The Apocrypha: Vehicles says, that if a weapon is twin-linked, I should simply add their Clip and Rate of Fire values when firing them from a vehicle. Therefore, in this case a twin-linked Heavy Stubber, has an effective RoF = 20 (10+10) on full auto.
A question: why 20 shots are better than 10 shots, if the maximum theoretically possible amount of degrees of success in Dark Heresy (which determine the amount of hits) cannot exceed 10?