From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156712-373317
Question: Can I add insecticidal soap to a fungicide spray such as
sulfur, so I can "kill two birds with one stone," so to speak? Or must
they be used separately?
Answer: It's not a good idea to mix pesticides. There might be a
chemical reaction between the compounds, or the combination could be
phytotoxic (react with the sun to burn leaf surfaces). Be sure to
identify the cause of any problems you are having, then choose the
proper pesticide for the job, following label directions carefully.
There are some commercial mixes available; these have been tested for
safety.