I have a similar situation. I only have two cats and two dishes but they seem to empty at different rates. As far as I can tell (or see) they each prefer their own dish but I have seen them eat from each others. I usually put new kibble in the empty dish and pour the old kibble on top and they usually eat it. I think the fresh smell underneath is a motivator. Of course the food is never really OLD. Maybe a day or two. I've been blessed w/ two good eaters that don't beg for people food and are happy with what's in their bowl. I hardly every have to throw away food unless it's the bottom of the bag and it's "dusty" w/broken kibbles that makes them hack a bit.
Claudia