With 40 virus scanners it would be completely unmanageable. So you'll constantly be clicking on 'shut up, it's okay!' dialog boxes and pop-ups. Remember, although you marginally increase your chances of finding actual malicious programs by running more virus scanners, you also dramatically increase your chances of false positives.
This would drive CPU through the roof when doing writes to disk, and you would be getting loads of false positive results, but it'd work. If you were super paranoid, you could install numerous virus scanners and let them all do 'on-access scanning' at once. On the other hand, actual viruses that are malicious will only be detected by some of the virus scanners, while not others. You will notice that, if you use certain websites that run your upload through 40+ virus scanners, almost every executable you can find will flag something in at least one virus scanner. It may run slower it may run much slower - but it'll work. if you are using the latest version of Windows Defender combined with the latest version of any self-respecting antivirus program from a third party.
Recent versions of Windows Defender in Win7/Win8, being built into Windows, are explicitly compatible (thanks to changes made by other antivirus vendors, as well as changes made by Microsoft themselves) with most third-party virus scanners.
