Although you don't physically have to, but it can cause conflict if an infected file was detected that both recognised they fight for control of the action. As one scanner tries to open a file to scan, avast's resident scanner intercepts that and scans the file first, this causes duplication of files scanned and can seriously slow the overall scan duration.
I would suggest you disconnect from the internet so there are no unwelcome distractions, emails received that need scanned, possible port scan activity which may trigger either your firewall or network shield if it detects any malicious intent. All of these would reqiure CPU cycles and pause the running scan or may be themselves paused awaiting the on-demand scan to release CPU time.
