It isn’t an issue of conflicting with avast, but two resident anti-spy/malware applications installed. The same general rule would apply for those as we apply to resident anti-virus applications, only on installed.
I have avast! plus SAS Pro and MBAM free (because I had SAS Pro before MBAM came out) on this system. On my other system I have avast and MBAM Pro, no SAS, never got round to installing it and if I do it will only be the SAS free version, to avoid possible conflict.