I think Windows Defender in Windows 8.1 is very good one for free anti virus.I prefer avast too as best free Antivirus.Is there any problem of using both of them simultaneously?
It "may or may not" create a compatibility issue. You might have to turn Window Defender - off.
Computer>C:>Program Files>Windows Defender>>MSASCui>Tools>Options>Uncheck Real-time protection and Automatic Scanning.
Windows Defender in windows 8.1 itself is best and it may not require third-party security programs.
If you install any other Anti-virus in Windows 8.1, It will automatically disable other running external programs in case of a compatibility issue.