certain antivirus softwares will not install if existing antivirus software is found, one must be removed. but if you can have 2 then its good for you, by the way why do you wanna have two antivirus softwares in your pc? one is not good enough for you? just curious
What happened with me was that every time you started the computer, you would get a message saying some things in Avast have been turned off because of Norton.