Let's first see if I could guess what you actually meant to say
"Is it possible to play different audio on separate encoders in the same SAM instance?"
- No, that's not possible. All encoders are sourced from the very same audio input (either a soundcard or SAM's internal DSP stream)
However as you were asking about Shoutcast in particular, there are options to insert ads server-side or put a proxy/man in the middle in between you/SAM/your encoder and the stream server, which could insert ads specific for just one server/mountpoint.