It is a rule of thumb not to have two RTP anti virus's running at the same time. This can cause conflicts,crashes, and in some cases leave you even more open to threats.
However there has been exceptions to this rule as some software companies purposely make their software compatible with each other.
As for spybot this typically plays with most anti virus programs with issue and is good to have installed. So something like Avast + Spybot would do quite well.