I have been asked to re-engineer an existing demo from a CD. I do not have the master stems to do the mix in an easy fashion, only the final .wav recordings. Is there a way to separate the recorded tracks to re-mix the songs for the band? Or, do I need to contact the original engineer?
No, the simple answer is there is no way to separate a stereo mix into its component tracks.
You either have to get them from the original engineer as you suggest, or see what you can do with EQ and dynamics processing to improve the mix, where the options obviously are far more limited.