Here's the reason why multiple AV programs are not recommended:
1) Conflicts of real-time drivers, i.e. each driver will fight for getting first access to the file that needs to be scanned
2) Boot up time conflicts
3) Even when running a full system scan from one AV, there is a possibility the real time guard of the other AV will intercept these calls and scan the files that are being scanned by the on-demand scanner before the on-demand scanner actually scans it. This leads to scanning delay as well as crashes. Most real-time guards are now configured to exclude most on-demand scanners from reputed companies, but still, there could be a possible delay if say a new version was released etc.
So you see, its not just a problem of resources, it will cause problems with virus detection, disinfection, removal and even scanning.