I have observed a very funny fact about norton and mcafee antivirus programs.
If you install them both they detect each other as harmful programs.

Antimalware, by its very nature, needs to do things that are similar to the things real malware does.

This is probably intentional since having two AV systems installed at the same time causes continuous locks on system resources. As a historical example, both of these products have a scan on access feature that used to be triggered by the other products scan on access causing an effective DoS.