I would have them clean up their own spit every time they do it. After they are done cleaning it, they can have a time out. If they spit on time out, then the time out starts all over. If need be, take away a favorite toy when they spit and give it back when they can behave better. There is no reason for kids to be spitting like that, or at all. Make sure to talk to the parents about that one. I'm assuming you have already, but if you haven't, fill them in. That's just gross-YUCK!
Is the 2 year old copying the 3 year old or do they do it independently of each other? How do the parents handle it or do they claim it's not an issue at home?
Oh, I just remembered DS's 3 year old preschool teacher....she gave the kids a stamp on each hand at the beginning of class. I can't remember her method, but if the kids were naughty, then a stamp was washed off. It was a big deal to the kids to lose a stamp. Whenever it happened, DS always told me about it
Maybe you could do something like that. Or if there's no spitting from drop off til x time, they get a stamp. Then another stamp if no spitting from x time to x time, etc. Personally, I'd go with taking the stamp away. I think it might be more effective with your ages.