----Microsoft recommends that you have only one anti-virus program installed on your computer."If we have more than on AV running on the machine you would run into performance issue. As both we take resources. There is no harm in haveing two AV, but I feel it is not a wise move
This is what Symantec says http://www.symantec.com/business/support/index?page=content&id=TECH104806
Symantec strongly recommends that you run only one antivirus program at a time. Having more than one antivirus program active in memory uses additional resources and can result in program conflicts and false virus alerts. Your best defense against computer viruses and malicious programs is to keep your virus definitions up to date, and run Norton AntiVirus Auto-Protect. If you choose to install more than one antivirus program on your computer, then only one of them should be active in memory at a time. (The Norton AntiVirus module that is active in memory is named Auto-Protect. Other antivirus programs will have different names for this function.)
Good article to read
http://www.security-faqs.com/why-shouldnt-i-install-more-than-one-antivirus-program-at-a-time.html
http://www.worldstart.com/more-than-one-antivirus-good-or-bad/