The best explanation as to why you should NOT have more than one antivirus is: (same thing applies to firewalls)
"Most people know they should only have one but do they know why?
Having more than one AV would catch more viruses – right?
No, WRONG! Antivirus programs clash with each other, a bit like two guys fighting over one chick.
AV programs work at a low level whereas other programs run at high level. It is ok to have multiple instances of, lets say, web browsers as they just connect to the net then close down completely when you are finished surfing and they operate on the higher level.
So what actually happens is when you have two programs working at low level is they trip over each other causing program crashes and system lockups.
Most antivirus programs operate in two modes:
Scan Mode: the program examines the memory and files on your hard disk for traces of malware. This involves examining the contents each file for traces that ‘look like’ viruses.
Monitor Mode: usually called or referred to as ‘real time monitoring’, the program runs constantly scanning files as they are downloaded to your machine, notifying you if the file you just received contains something that appears to be a virus.
It’s ok to scan with a different antivirus program than your normal resident scanner as all that happens is the program will scan, report it findings then it will close down. Online scanners are good for second opinions. But if you install a second AV program it will (or should) inform you that another antivirus program has been detected and to uninstall it, if you choose to ignore this warning it will then install its own ‘real time monitors’ and that is when the problems will start – lockups, poor performance and crashes."
http://www.bleepingcomputer.com/forums/topic186533.html