If both are on the computer they attack each other, at least that is my experience. I did fine with Avast until I put Malwarebytes in there. Each seemed to identify the other as malware. My son all that out and loaded Mbytes with AVG and it has never given me any trouble. I have the paid versions of both now. One was 9.95 and the other was 14.99.
My son who is a bona fide qomputer whiz first installed Avast for me then later, when it got all balled up after I got Mbytes for a particular intruder that Mbytes was specified for, He cleaned up my box and said the two programs conflicted.
Sometimes such a conflict or issue can show up for a short window, but the maker fixes it quickly.
Just about all security software gets a bad update at some point, where they will show a false positive, but it gets fixed quickly.
Malwarebytes and Avast do fine with each other.