If the one is a resident anti-malware solution like avast it can be accompanied by a non-resident solution like MBAM, Voodooshield etc.
Two resident AntiVirus solution on one and the same system is a bad idea, as they are going to interfere like two dogs fighting on the same porch they are considered to defend against threats from outside. Competing resident av solutions are going to alert each other's definitions and create false alerts etc.
So always first establish what programs will fit together. When you have avast resident solution it disables resident Windows Defender to an extent, that should be. Check your AV from time to time you have all your protection elements still enabled like avast Web Shield for instance.
avast, putting security on the map again,
polonus (volunteer website security analyst and website error-hunter - using avast for 15 years now)
