After going through the Simplecast screencast and learning how to stream on both Sandbox A and B at different levels, I returned to NiceCast and was baffled how to do it on Mac. I wrote to Rogue Amoeba Software Support on Lee's suggestion. Here is my question and their answer:

Question: In Simplecast or SAM on my XP machine, I can broadcast using on two channels, one for people with higher bandwidth and one for people with lower bandwidth. In Nicecast, I can set up the different channels as different servers, but I don't see how to broadcast on two at once and how to do it with different quality settings????

Answer: Unfortunately, Nicecast does not support simultaneous broadcast of two streams as of yet. Maybe in the future.