In the past anti virus programs heavily conflicted with each other often detecting the other as a virus and attempting to quarantine it.
A second reason was that they are resource intensive and having two would slow your system down.
A third reason was that one AV can cause the other AVs scanning engine to fail to detect because it keeps interfering with it.
These days AVs can detect each other, many can work alongside one another. You can run Avast with Windows Defender no problem.
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