Saw this posted in the solution on TA. Maybe it applies?
For Possession + Shock Jockey you'll have to Possess the enemy first then shock them repeatedly untill the Possessed enemy gets a kill. The Possessed Enemy HAS TO GET THE KILL, NOT killing the enemy with Possession and Shock Jockey.
For Possession + Devil's Kiss you'll have to Possess the enemy first then Devil's Kiss setting them on fire, the Possessed enemy HAS to get the kill while being on Fire. This is a bit tricky since you could kill the Possessed Person with Fire Damage or it delays the kill. The Possessed person has to get the kill.