Never have more than one piece of antivirus software installed. It will hog system resources and cause conflicts/issues between different applications.
To clarify on the running two virus scanners at the same time:
Two or more installed is fine.
Two or more running at the same time is a big no-no for reasons already stated.
Alternatively, install one, do a full scan. If everything clear, uninstall it and install another one, then scan with that one.