i have Avast, NOD32
This can be a problem if they are both running together in real-time.
The concern with using more than one anti-virus program is due to conflicts that can arise from them both running together in real-time protection mode. Each program will often interpret the activity of the other as a virus and there is a greater chance of them alerting you to "False Positives". Further anti-virus software componets insert themselves into the operating systems core and using more than one can cause instability, crash your computer, slow performance and waste system resources.
While operating in real-time mode, if one AV program finds a virus and then the other AV program also finds the same virus, then both programs will be competing over exclusive rights on dealing with that virus. Each piece of AV software will attempt to seize the offending file and quarantine it. Further, if one AV finds and quarantines the file before the other one does, then you encounter the problem of both AV's wanting to scan each other's zipped or archived files and each reporting the other's quarantined contents. This can lead to a repetivite cycle of endless alerts that continually warn you that a virus has been found.
In contrast, using more than one anti-spyware program running in real-time mode simultaneously increases your protection coverage without causing the same kind of conflicts or affecting the stability of your system. No single program on the market can claim 100% detection/removal so using more than one is recommended. Each vendor has its own definition of what constitutes spyware and scanning your computer using different criteria will yield different results. The fact that each program has its own definition files means that some malware may be picked up by one that could be missed by another.