hmm...

so, are you using any sort of emitter? Or are you placing particles directly. If you're emitting from curve or something like that, you should be able to tell the different "emitters" to use the same particle object. I would think that would cause them to interact with each other.

I've not tried what you're attempting, though, so I'm not sure.

I have seen someone do a pretty good job with saliva strings and what he did was use actual soft-body objects with a saliva shader in the addition of blobby particles as the "spit" that drips off and shoots out of the mouth.