Hello,
This varies depending on what algorithm you are using so there is no one-size-fits-all answer. If you had a question about a specific algorithm, that would be easier to answer. There are a few general things to keep in mind:
– Dry stereo signal is always preserved
– Delay algorithms and Pitch algorithms divide stereo signals among A and B voices
– Most reverbs don’t do an explicit sum, they feed a stereo reverb tank, just a like a room