It's not additive, but it does mean the other civ that finished patronage now also suffers the effect. That does mean if two different civs finish patronage, the finisher has no added effect for any other civs that finish it, unless one of the other civs that finshed patronage is subsequently eliminated from the game of course. I'd like to see the finisher modified slightly to be "other civs that have not finished patronage suffer faster decay, but your civ is exempt from any other civs faster decay"
EDIT -- this is based on my experience with United Front; let me be clear I don't know definitively after the new patch but I assume it works the same.